Apart from the official client found at the bitcoin.org site what is the list of alternative bitcoin clients? And what OS do they work on?
There is a current list of clients here http://en.bitcoin.it/wiki/Category:Clients
There is development discussion around alternative clients here http://bitcointalk.org/index.php?board=37.0
Currently the "Official Client" (more specifically, the Satoshi client) is the most widely used and has implementations that run on Windows, OS X, and Linux. However, multiple alternative clients have been developed, including ones that run on Android, and this is currently a high priority for the bitcoin community.
The current most popular alternative clients are BitcoinJ, written in java http://code.google.com/p/bitcoinj/
and BitDroid, which runs on Android https://github.com/cdecker/BitDroid-Network