1785 - Get Him to the Greek

I'm actually not sure if I've seen this film since it was in theatres, but I remember (like Sarah Marshall) being surprised just how hilarious I found it to be, and what a fun film.  It's a great addition to the buddy comedy and while not quite a rival for Planes, Trains, & Automobiles it's definitely a close relative.  The soundtrack to this film (almost entirely - if not absolutely entirely original songs by the main character) is something quite spectacular and elevates the film from being merely good to pretty nifty-great (that's now a term - use it!).  Brand and Hill are a great pairing and their energy feeds off each other in really delightful ways and shifts nicely when they're with Bryne and Moss as their respective love interests.
If you've never seen this and are a fan of any of the cast or just the buddy comedy in general this is a must see - and the songs are an awesome bonus - I listen to the soundtrack fairly regularly.