Must-have apps:
Web:
Firefox
Open Source Office:
NeoOffice
FTP:
Filezilla
FireFTP plugin for Firefox is also excellent, and works through a SOCKS proxy
IM:
Adium
Virtualization:
VirtualBox (Open source--not as fancy, but solid)
VMWare Fusion (Now with DX9 support, but w/ a Macbook not really important)
Misc:
Quicksilver (Launcher, make-life-simple tool)
Caffeine (Lets you easily toggle the screen dimming/sleep function)
Dropbox (Sync between computers, plus online access/backup, 2GB free)
VLC (Multimedia player for just about everything)
If you're going to run Virtualization, definitely get 2GB of Ram or more, especially given that it's so cheap and so easy to install. (Even if you're not, running virtual machines, I still recommend it, you'll notice an improvement.)
If you're still working with PCs, Microsoft also has a RDP client for Mac that's free and works OK.
Just a sidenote; have you tried out OpenOffice.org 3rc1? Pretty solid native OS X version of OOo. I'm really digging it.
And I echo the VirtualBox and Fusion recommendations.