First stuff I installed on my Nexus 7
* Flash - had to side-load as an apk, but it's still needed on Android, no matter what Adobe say.
* Firefox Beta + Adblock Plus + Phony - stomps mightily on Chrome on the Nexus 7. Set Phony extension to desktop version to avoid getting those daft mobile versions on a tablet, plus it then lets you go to www.bbc.co.uk/iplayer/ and view BBC progs just like you do on the desktop (yes, in HD too).
* Astro File Manager. I've been familiar with this one the longest, but no doubt other FM's mentioned are equally as good.
* MX Player. This is my favourite video player on the HP Touchpad (CM9) and it's just as good on the Nexus 7.
* Winamp. Trhey may have moved some features to the Pro version after offering them in the free version (which I think is a really bad PR move), but even the free version is pretty good.
* Camera Launcher for Nexus 7. In a moment of madness, Asus/Google decided not to present the built-in camera app to the end-user. This app basically makes it "visible" again.
* FBReader. Best Android e-book reader out there (much better than the Kindle app for example).
* Cover Art Downloader. Put in on my Nexus 7 out of habit - nice to have artwork with your MP3s.
* Advanced Task Killer. I put the widget on my home screen and press it when I'm about to play a game.
* Box (or Dropbox etc.). Useful app to mess around with your cloud storage (I've got 50GB, but I don't use it much because I don't have Net access on the move with the Nexus 7 except via expensive/data limited 3G tethering with my Android phone).
Of course, once I'd done the above, the next step was unlocking, rooting and putting on a Modaco ROM (yay for landscape rotation on home screens), but that's a bit off-topic I guess.