Canon, Sony, Casio and Olympus all make housings for their most popular 5MP compact cameras. You can get Ikelite and other 3rd party housings for most other cameras but they cost much more (economies of scale I suppose). But the 3rd party housings are usually more rugged than the camera maker's housings.
That said, it matters what's important to you. Some people don't like proprietary batteries and only want cameras that use AA's (NiMh rechargeable). Some people want to use CF memory instead of SD memory or vis-versa.
I favor Canon cameras in general because of the high quality of Canon glass and their Digic II processor offers the clearest image in the consumer/prosumer category (though their most recent powershot SD cameras seem to be not as sharp as their A or S series cameras).
With that in mind, I recommend:
1. Canon A96 with their WP-DC50 housing
2. Canon S500 with their WP-DC800 housing (even smaller than the A96 but no manual controls)
3. Canon S60 with their WP-DC40 (great manual controls but more expensive)
Canon makes even smaller cameras, the S10 and S20 but the AW-DC10 housing is only rated for 10ft (snorkeling depth), whilst the other housings I recommended are all rated for 120ft. -132ft.