As with any true Mac geek, I need to keep up with the latest software available for the Mac. There are a bunch of sites that cater to this need, which I visit more often than I would care to admit.
VersionTracker - Pretty much the only site you need to keep up-to-date on the latest software.
Apple Software Updates - Updates to Mac OS software from Apple.
There are many other sites out there that cater to Mac software, but I don't use them regularly, or don't feel they deserve to be on this list :-)