Once again, lack of standards shows up here. There are quite a few Tamil keyboard layouts, the traditional typewriter keyboard; then with the surge of internet arrived the romanized transliteration keyboards; later the TAmil-Nadu government played its part by prescribing a tamilnet99 keyboard. These are only a few to talk about; we have a few others which do not fall into any of these "standards."
There are two Tamil keyboard drivers for the X Window System, both of them set to tamilnet99 standards (see tamilnet99 website for the details on the keymap). You will be able to download both the keydrivers from the Yahoo! tamilinix group files section .
The first driver is tamil_kmap, created by Vasee. It is based on the original version of Siva. It is operable under both TSCII 1.6 and TAB encodings. The detailed installation instructions are given in the README file in the package. It is very simple to install. First, untar the package into a temporary directory. Then type:
cp ta /usr/X11R6/lib/X11/xkb/symbols/
then: cp Compose /usr/X11R6/lib/X11/locale/iso8859-1
and put the shell script setkb into a directory on your system PATH . You may need to become root to copy these files into these directories.
To use the Tamil keyboard, type setkb tscii or setkb tab. From inside the keyboard driver you will be able to switch between the two standards, and also between Roman and Tamil fonts.
The other keyboard driver, tamilvp (vp for Visaip Palakai) is written and maintained by Dinesh. As indicated above, you may download that from the Yahoo! tamilinix group file section. It is available as rpm (I have not tried it out yet). Just install the rpm and files will be in appropriate locations. To run the program type tamilvp and you will get the GUI cell to choose between Tamil (TSCII 1.6 or TAB) and English.