TamilKey is a free Firefox extension that, when activated, allows you to type using Tamil language characters. While we can't attest to its accuracy, it worked for us.
Once you download the extension, you'll find it under your Add-ons menu, and in your context menu. The Options menu, with the exception of the hotkeys, is all in Tamil. You can set your own hotkey combos, but using the default settings, we activated the program, and sure enough, when we started typing in our browser, the characters appeared. The only drawback to the program is that when you deactivate the program, you have to restart the browser. A deactivation hotkey combo would have been more convenient, so as to not interrupt browsing.
Obviously, if you're not familiar with Tamil, this extension isn't for you, but if you are, its a handy tool to have on hand to switch back and forth between languages.
Tamilkey is a utility for typing in Tamil language. It will be ideal for Tamil bloggers. Tamilkey will support most of the popular keyboard layouts used among Tamils. Currently Tamilkey supports Anjal, Tamilnet99, Bamini, and Old and New Tamil typewriter keyboard layouts.