Besides its Linux features, there often are some main features which have to be considered when buying a laptop. For Linux features please see Part V in Linux on the Road Hardware In Detail below.
Don't underestimate the weight of a laptop. This weight is mainly influenced by:
internal components, such as CD drive, floppy drive
material used for the case, usually they are either from plastics or from magnesium.
Recent laptops come with active matrix (TFT) displays. Laptops with passive matrix (DSTN) are no longer manufactured. Active matrix displays have better color and contrast, but usually cost more and use more power. Also consider the screen size. Laptops may be purchased with screens up to 17". A bigger screen weighs more, costs more, and is harder to carry, but is good for a portable desktop replacement.
The available battery types are Lithium Ion (LiIon), Nickel Metal Hydride ( NiMH) and Nickel Cadmium (NiCd). Though almost all current laptops come with LiIon batteries.
LiIon batteries are the most expensive ones but a lot lighter than NiCd for the same energy content, and have minimal - but present - memory effects. NiMH is better than NiCd, but still rather heavy and does suffer some (although less than NiCd) memory effects.
Unfortunately most laptops come with a proprietary battery size. So they are not interchangeable between different models.
For details about systems which are supported by the Linux Kernel, see the The linux-kernel mailing list FAQ.
i286: Linux doesn't support this CPU family yet. But there are some efforts at ELKS. If you like, you may use Minix, which is also a free Unix operating system. Minix supports 8088 to 286 CPUs with as little as 640K memory. Actually there are some laptops with ELKS and MINIX around.
i386: This covers PCs based on Intel-compatible processors, including Intel's 386, 486, Pentium, Pentium Pro and Pentium II, and compatible processors by AMD, Cyrix and others. Most of the currently available laptops use Intel compatible CPUs and have quite good Linux support.
m68k: This covers Amigas and Ataris having a Motorola 680x0 processor for x>=2; with MMU. And the early Apple/Macintosh computers.
There was a long series of Apple PowerBooks and other laptops based on the m68k chip. Macintosh Portable (an ugly 16-pound first attempt); PowerBook 100, 140, 170, 145, 160, 180c, 165c, 520c, 540c, 550c, 190; Duo 210, 230, 250, 270c, 280. The PowerBook Duos were available at the same time as the PowerBooks, they were a sort of subnotebook, but were designed so that you could plug them into a base station (a DuoDock) with more RAM, peripherals, etcetera, so that they could also act as a desktop computer. The first PowerPC PowerBooks were the ill-starred PowerBook 5300 (after the 190) and the Duo 2300c.
The proper place to go for information on running Linux on m68k Macintoshes is linux-m68k.
"Much like laptops of the Intel/Linux world, Mac laptops have generally different setups that can be very hard to figure out. Also, because of a general lack of machines to test, we are only aware of boots on the Powerbook 145, Powerbook 150, Powerbook 170, Powerbook 180, and Powerbook 190. Even if it boots, we currently have no support for Powerbook-style ADB, the APM support, or just about anything else on them. This means the only way to log in is with a terminal hooked up to the serial interface, this has been tested on the 170."
"Several Powerbooks have internal IDE which is supported. PCMCIA drivers will be forthcoming if someone can supply the necessary hardware information to write a driver. As always, an FPU is needed also. Many of the later models have the 68LC040 processor without FPU, and many of these processors are broken with respect to the FPU trap mechanism so they can't run regular Linux binaries even with FPU emulation. Current status on Powerbooks 140, 160, 165, 165c, 180c, 190, 520 and Duos 210, 230, 250, 270c, 280, and 280c is unknown."
Also there are two Atari laptops, for which I don't have enough information. The following quotations are from the Atari Gallery.
"The STacy was released shortly after the Mega ST to provide a portable means of Atari computing. STacy computers were shipped with TOS v1.04.
Designed to replace the STacy as the defacto portable ST computer, the ST Book brought the basic computing power of an ST to a lightweight notebook computer. This machine was only released in Europe and Atari only shipped a very small quantity. The ST Book was shipped with TOS v2.06."
From Stok, Leon <stok_AT_YIS.NL>: The STacey and the ST Book, both can't run Linux since they are only shipped with an 68000 CPU, which doesnt have a MMU unit.
As far as I know Amiga has never produced laptops. One company manufactured kits to convert desktop Amigas to portables. These used regular Amiga motherboards so any Linux setup that supports the regular Amiga setups will support these.
Alpha, Sparc, Sparc64 architectures: These are currently under construction. As far as I know there are only the Tadpole SPARC and ALPHA laptops, and some other ALPHA laptops available. NatureTech offers also SPARC CPUs in laptops. The TuxMobil survey of Solaris on laptops and notebooks may also be helpful.
StrongARM: a very low-power CPU found in Rebel.com's popular NetWinder (some kind of mobile computer, too), and actively supported in the Debian project, it is also in several WinCE machines, such as HP's Jornadas. Only the lack of tech specs prevents Linux from being ported to these tiny, long-battery-life machines. A full-scale StrongARM-based laptop would make a superb Linux platform.
For PDAs with ARM/StrongARM CPU see the Part II in Linux on the RoadHandheld Devices part below.
MIPS: Used in SGI mainframes and Cobalt Micro intranet appliances, chips based on this architecture are used in many Windows-CE machines. Linux has been ported to a few of these.
64bit CPUs: At TuxMobil there is a survey of laptops with 64bit CPUs .
At higher speed, a CPU consumes more power and generates more heat. Therefore, in many laptops a special low-power CPU is used. Usually, this special CPU doesn't use as much power as a similar processor used in a desktop. These special CPUs are also more expensive. As a side effect you may find that laptops with a desktop CPU often have a quite noisy fan.
Laptops and notebooks are often described by the number of spindles.
one spindle: harddisk. Usually sub-notebooks, often provided with an external optical drive (CD/DVD).
two spindles: harddisk, optical drive (CD/DVD).
three spindles: harddisk, optical drive (CD/DVD), floppy drive. These laptops are often used as desktop PC replacement.
An enormously important issue. Anything based on PPC or Pentium will generate enormous amounts of heat which must be dissipated. Generally, this means either a fan, or a heat sink the size of the case. If it's a fan, the air path shouldn't get blocked, or it will overheat and burn out. This means machines with a fan mounted in the bottom are a big, big mistake: you can't use them on a soft surface.
Though you might use your desktop computer to do longer writings, a good keyboard can save you some head- and fingeraches. Look especially for the location of special keys like: <ESC>, <TAB>, <Pos1>, <End>, <PageDown>, <PageUp> and the cursor keys.
Laptops are quite expensive if you compare them with desktops (though maybe not if compared with LCD, IrDA®, PCMCIA capabilities). So you may decide between a brand or no-name product. Though I would like to encourage you to take a no-name product, there are some caveats. I have experienced that laptops break often, so you are better off, when you have an after-sales warranty, which is usually only offered with brand products. Or you may decide to take a second hand machine. When I tried this, I discovered that the laptop market is changing quite often. A new generation is released approximately every three months (compared by CPU speed, harddisk capacity, screen size etc.). So laptops become old very quick. But this scheme often isn't followed by the prices for second hand laptops. They seem too expensive to me. Anyway if you plan on purchasing a second hand machine, review my recommendations on checking the machine.