Apple has freely released its Sangam MN fonts present on the iPhone 4 and iPad.
I have created a font for Mac users who wish to read and write in Sinhalese (සිංහල). The font is a Mac version of Malithi Web, a typeface by Pushpananda Ekanayaka. It's not yet finished, but is being released in the present workable state and users are requested to give feedback, especially if you find bugs.
To install, put the font in ~/Library/Fonts/
Once you have installed the font, you may then change your browser setting to request Sinhala 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 Sinhala, which will be near the bottom of the list that appears.
The font works with any version of Mac OS X. You can also download the morph input file for Malithi Web if you so wish.
Problems? If you are not sure if the font is installed correctly, or it doesn't seem to work, go to http://si.wikipedia.org/ with the Safari browser (OmniWeb/Shiira/iCab 4 are fine, but not Firefox or Opera) and check if you see correctly formed Sinhala. If you see squares and not text, then the font hasn't been loaded by the OS. Check it's in the right place (~/Library/Fonts) and reboot your machine if all else fails. If you do see characters, but the vowels are not getting their positions swapped (appearing in front of the consonants), then it's likely you have OpenType Sinhala fonts installed and they are getting chosen by the computer instead of Malithi Web. The only thing you can do is disable these fonts with Font Book, or remove them entirely.
Copyright Pushpananda Ekanayake, unlimited distribution permitted.
Thanks to Hari Prasad Nadig for invaluable help and guidance with the Kannada language and script, given whilst I created a Kannada font, and which was useful in this project too.