It makes me happy when one of my first and favorite Tamil language resources links back to me! Azhagi, which is a transliteration and writing software for Tamil has a page of resources and I got a mention. Hooray!
In all seriousness, although the Google transliteration is fast and easy since it is built in to so many Google products, it will sometimes auto-correct a word and then use what is actually an incorrect spelling. I have never had that problem with Azhagi. Azhagi also has more functions than the Google transliterate function; namely that it has a reverse transliterate, which can put Tamil characters into Roman characters. It also allows you to save your work as an rtf file and print it.
Go to their downloads page to install the program and get Tamil fonts.