I'm constantly looking to make my computer better, so I love free utilities to do the job for me. I use tons of them, like Ad-Aware SE Personal to keep out all those nasties floating in the ether (I don't have a Mac you see). PC World magazine has listed 101 of these wonderful finds in one place. From Ajax 13 to Gmail (which you must use) to Zoho you'll find more than you need.
Read the 101 list here.