OK, here is my personal list of 5 bits of software you should download now. They are all free and unrestricted:
1) VLC -
http://www.videolan.org/vlc/ - this is a media player that allows you to play almost anything. Doesn't restrict you to DVD regions like DVD Player and will allow full screen (goodness know's why Quicktime doesn't - seems like a rip off to me)
2) iStumbler -
http://www.istumbler.net/ - lets you find available wireless networks. Don't steal wifi though...
3) Handbrake -
http://handbrake.fr/ - lets you rip DVDs onto your hard drive so you can watch them without having to lug discs around. Don't pirate them though, that would be wrong
4) Firefox -
http://www.mozilla.org/download.html - not as pretty as Safari but if you are used to it, you can use it on Windows or Linux too. Highly extensible
5) Gimp -
http://www.gimp.org/macintosh/ - Macs like to bang on about how great they are for graphics. They don't come as standard with anything that can edit photos. Gimp is like free Photoshop
For many other recommendations, try
http://www.opensourcemac.org/