'Azhagi' has been existing since 2000, helping 1000s of users worldwide to transliterate/type in Tamil, both as a stand-alone application and also to directly input in ALL windows applications (offline or online). Later, Azhagi+ (AzhagiPlus) was also released in 2011 with very unique/special features as listed here. 'Classic' Azhagi also hosts a few 'unique' Tamil transliteration tools, most of which have been available since 2000 or early 2000s itself. They are explained here.

Both Azhagi+ and Azhagi host a 'pathbreaking' Tamil transliteration scheme, which has remained peerless* since 2000. Ofcourse it may be bettered by any other entity in the future. But, as of now (2016), as far as I know, it is still peerless, providing the most intuitive/natural, easiest and fastest Tamil transliteration experience to the users. Azhagi's abovementioned unique and innovative Tamil transliteration scheme has been explained in a nutshell:
here
and in detail:
here (by the editor of mazhalaigal.com)

(*) This uniqueness of Azhagi is amongst non-freeform transliterators (for details on free-form transliterators, click here) only.