Congrats on making the switch, i did it myself last month
Here is some stuff I have that i find pretty useful
Adium - icq/msn program
Openoffice - microsoft office basically
Entourage - for e-mail/tasks/calendars
Air Video Server - lets you stream any format movie on your mac to your iphone to watch
Vuze - for torrents/p2p
VLC - for avi movies
Filezilla - for uploading/downloading stuff to servers
Hope this helps