Great thread! Now I have to think of what planes one might spot!

F70 (rare)
F100 (rare)
737-500 (rare)
737-300
737-400 (uncommon)
737-400F (rare, try Alaska Airlines)
737-400Combi (rare, try Alaska Airlines)
737-600 (rare, try Westjet)
737-700
737-800
737-900 (rare, try Alaska Airlines), -900ER (more common)
747-400
747-8
757-200
757-300 (rare, try DL and UA)
767-200ER (rare, try US)
767-300ER
767-400ER (uncommon, try DL)
777-200 (rare), -200ER (common), -200LR (uncommon, try DL), -300ER
787-8
787-9
717 (uncommon, try DL)
A318 (rare, try BA at JFK)
A319
A320
A321
A330-200 (uncommon, try DL)
A330-300
A340-200 (rare, maybe Air Tahiti Nui at LAX????)
A340-300 (a few Virgin Atlantics)
A340-500 (rare)
A340-600 (not that common, try LH)
A350 (rare, just released)
A380 (not that common)
quite a few commuter jets, like the Canadair series, Embraer ERJ135/140/145, and Embraer E170/175/190/195.

Luckily, I can brag that I've been on a flight aboard the 747SP, L1011-500, BAe RJ85, Fokker F70, MD-11, and BAC One-Eleven-500.