Asking which apps you need without telling us what your needs are is a bit of a shot in the dark don't you think?
My app set I have installed on EVERY Mac I own:
MS Office
iWork
Skype
Dropbox
Omnigraffle Professional (I suck at freehand diagrams, great stencils!)
VLC
Merlin (Project Management software for OSX)
RapidWeaver (website creation tool for OSX)
Handbrake
Final Cut Studio (mainly for renders, not for my MBA's)
Photoshop (not the whole Creative Suite, just PS)
I also keep a copy of Unison (Usenet reader) on all my Mac's but don't use it that often.