Here are a few of my favs:

The Office/Little Miss Sunshine. Outstanding, Soldier. Toby/Frank.
This stole my heart and will forever be my head-canon for Toby Flenderson.
http://sundancekid.livejournal.com/535837.html

The Office/M*A*S*H. O*F*F*I*C*E. canon pairings.
I almost skipped this one because I've never watched MASH (heard about it from my parents haha) but this is really good. I wish there were more.
http://annakovsky.livejournal.com/173740.html

Inception/Sherlock. Nothing We Could Judge. Arthur/Eames, Sherlock/John.
http://green-postit.livejournal.com/108442.html

Fight Club/Harry Potter. The Craziest Person I've Met This Week. Narrator/Remus.
http://community.livejournal.com/crossover_hp/13990.html

House/Grey's Anatomy. Blame It Upon A Rush Of Blood To The Head. House/Addison.
http://mardia.livejournal.com/131270.html

Iron Man/Spider-Man. Paparazzo. Tony Stark/Peter Parker.
http://basingstoke.livejournal.com/361950.html
& sequel: Tabloid.
http://basingstoke.livejournal.com/391787.html