Problem: Some piece (or pieces) of rogue software have cluttered up your Open With contextual menu, which you can see by right-clicking or control-clicking any document in the Finder. This problem seems to be most prevalent with virtual machines that allow you to open documents with Windows applications, but tend not to clean up after themselves. After having both Parallels and VMWare installed on my MacBook Pro, my Open With menu was a mess.
Solution: Lucky for me, I noticed David Chartier's question about this on Twitter around the same time as I was wondering what to do about it. Some friendly person pointed him to a posting on Apple's discussion forum (also noted on Mac OS X Hints here and here), noting that running a specific command in a terminal window will rebuild your launch services, which repopulates the Open With menu with a current list of applications, without duplicates. It worked perfectly for me, but beware, on my system it took about 10 minutes to complete, and I suspect it could take more on a sufficiently gummed-up system.
Here's the Leopard version of the command (the path to the tool is different in Tiger, see here).
I broke it into three lines for readability, but the \ at the end of the line is bash-speak for "keep on going with the same command" -- you can copy and paste it directly and it should work, or if you type it on one line without the backslashes, it will also work fine. Just copy and paste it into terminal and it'll work just fine.
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain user
If, preferring to avoid the Terminal, you want a handy GUI app to rebuild the Launch Services database with a couple of clicks, check out Titanium's OnyX or Maintenance utilities, both free of charge.