Apple has freely released its Sangam MN fonts present on the iPhone 4 and iPad.
I have created a font and keyboard layout for Mac users who wish to read and write in Kannada (ಕನ್ನಡ). The fonts are Mac-compatible Regular and Bold versions of Kedage, a typeface by the Indian Institute of Science in Bangalore. Neither of them are finished, but are being released in the present workable state and users are requested to give feedback, especially if you find font bugs.
Update: Kedage now includes a bold face too, as well as some minor technical corrections.
Update: I have improved the handling of vyanjanas with multiple vattus, such as ಕ್ಷ್ಮ and ಕ್ಷ್ಯ.
To install, put the font in ~/Library/Fonts/ and they keyboard layout in ~/Library/Keyboard Layouts/. Turn the keyboard layout on in the International pane of the System Preferences, it will then be accessable from the Keyboard menu in the upper-right of the screen. Choose the "Show Keyboard Viewer" command in the same menu to display a keyboard layout, it will look something like this:
Once you have installed the font and keyboard layout, you may then change your browser setting to request Kannada pages first. Your browser is currently set to request United States English pages. Do this by going to the System Preferences, click on the International icon (United Nations flag), Clicking the Edit List… button and enabling Kannada, which will be near the bottom of the list that appears.
The font works with any version of Mac OS X, the keyboard layout requires Mac OS 10.2 (Jaguar) or higher. For further information about the font and keyboard layout, please see this help file (ಕೆದಗೆ ಫಾಂಟ್ ಸಹಾಯ ಪರಿವಿಡಿ), created by Basavaraj Pujar. They are both released under the GNU General Public License. You can download the morph input file for Kedage if you so wish.
Ashwin Kumar's alternative layout does not adhere to the INSCRIPT layout shown above, and is instead similar to QWERTY. Additionally, special modifier keys have been provided for easier input of Matras.
Thanks to Hari Prasad Nadig for invaluable help and guidance with the Kannada language and script.
See also: In addition to Kedage, there are Telugu and Sinhala fonts available from me for Mac OS X too.