I've rented this movie a few times and just had to add it to my collection.
Two British sci-fi buffs go to America to attend Comic-Con. They rent a motorhome and decide to visit Area 51 and a number of other sci-fi sites. After a mishap with a couple of red-necks, they end up on a dark highway where they witness a car crash. If you don't know the rest of the plot, read it on Wikipedia. Or better yet, just buy the movie on spec - you won't be disappointed.
The premise of the film is great. Simon Pegg and Nick Frost are the modern day Abbot and Costello - they play so well off each other. Paul, the computer generated alien that foists himself onto the pair, is hilarious. Sigourney Weaver was a great choice as the "bad guy". There are lots of references to many other sci-fi films for those into the genre, but the film works just as well as a comedy.
One of my favourite flicks.