On the mac there are quite a few options. Depends on how many images you process.
If youre just after something simple to use, iPhoto is great and free (comes with mac).
-Adobe Lightroom is one of the pro options, buts costs £200
-Aperture 2 is your other pro option but costs £115 (or just £55! if u can use the Higher Education discount)
(if youre a Nikon user theres also Capture NX 2, which handles Nikon RAW files better)
Either of these are good for post processing your photos, and useful if you have a dSLR and are processing RAW images. There are pros and cons of both Aperture and Lightroom, but they have trial versions so download them and try each of them out if u want.
Cant comment on either of them personally as Ive only just started using Aperture a little bit, but theres plenty of info on the web comparing the two programs .