The Dravidian languages, which form a group apparently unrelated to any other, are spoken in Southern India and in Sri Lanka. OS X 10.4 comes with support for Tamil, and you can find info on how to use it here. For typing in the other main family members -- Kannada, Malayalam, and Telegu -- you need to download and install fonts and keyboards, using either the commercial kits available from XenoTypeTech or the free stuff listed below:
For Kannada, you can get the Kedage font and a keyboard layout from Nick Shanks' site.
For Malayalam, you can get the Rachana font and a keyboard from the MacMalayalam Site.
For Telugu, use the Pothana font from Nick Shanks and the keyboard layout from my iDisk.
Web sites in these languages may still be using custom encodings instead of Unicode. In that case you will need to download whatever font they require (e.g. manorama for manoramaonline.com) and you may need to try different browsers as well.