You need an office suite so here are the contenders:
NeoOffice free
OpenOffice free
iWork paid
Office for Mac (MS stuff) paid
For multimedia:
VLC free
Perian free
Transmission (don't download illegal stuff) free
Quicktime paid
For Web:
Safari 4 Dev Preview (has a neat web app thing, can't say anymore due to the NDA stuff) register, but free to
FireFox free
Google Earth free
Skype free
If you do programming like I do:
Textmate paid
Textwrangler or BBEDIT free, later is paid
Editing multimedia (photos, movies, etc)
Final Cut Express (Studio if your a professional) paid
Logic Express (studio if your a pro) paid
Photoshop paid
Seashore/GIMP/Pixelmator (photo editor, similar to photoshop) free
Customization of the GUI (interface):
Candy Bar see below
DockIt not sure about price
App removers (you'll need these, only one though for uninstalling stuff)
AppClean free
AppDelete free
Appzapper (paid)