Here's the rundown of my F-keys:
F1: Gnus. Takes some learning, but cut my daily email time down to 20 minutes.
F2: ansi-term. Fairly full featured terminal emulator with a mode where you can switch back into behaving like an emacs buffer for kill/yank/macro goodness.
F3: slime. Crazy good tool for hacking the good hack with Common Lisp.
F4: switch-to-buffer "scratch". The scratch buffer is a good place to do fancy Emacs stuff when you don't want to type in the mini-buffer (try C-j).
F5: compile. You don't realize how great this is until you also grok next-error and previous-error.
F6: visits the buffer I have usually associated with an SQL session, either in an ansi-term or using psql.
F7: w3m. Very slick, fast web browser. Perfect for searching documentation.
F9: Visit my todo file in org-mode. org-mode, like gnus, takes a while to learn, but you can go so damn fast with it, it's worth it.
F10: calendar. Not as nice as org-mode but I haven't switched over all the way yet.
F11: open the EMMS playlist. EMMS doesn't come with the usual upstream Emacs distribution, but it's a pretty reasonable OGG/MP3/etc player.
F12: I run ERC and bitlbee. F12 brings me to the chat buffer with all my contacts from Google Talk and AIM in it.
Insanely cool thing not on one of my F-keys: TRAMP. TRAMP integrates into the way emacs finds, reads, and writes files. You can use file names like /sudo::/etc/passwd to edit things as root without starting another emacs, or /ssh:[email protected]:foo/bar.html to edit files remotely without staring a remote emacs.
Generalized ompletion modes and facilities are useful too. I like completion.el, but there are several to choose from.
Definitely check out www.emacswiki.org. Tons of good ideas there. Good luck!