This one is quite an advanced tip.
If you right-click on a file you get a little menu, and if you select ‘open with’ you get a list of all the programs you can open a file with.
After a while of installing new versions of programs, this list can become quite long!
To reset it open up a Terminal window. (from Applications/Utilities) and paste the following line:
/System/Library/Frameworks/ApplicationServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user
press return wait about 20 seconds, and your open with menu will be fixed.
I think this only works on 10.4 and it may reset some other things to, like what applications open certain files.