Mac/Windows: If you're not too keen on buying movies from the iTunes Store but would like to organise your movies in a program like iTunes, MetaX will save you the time of manually tagging everything by fetching metadata from the net.
Currently, MetaX can tag mp4, m4v, m4a, m4b, m4p and mov files (which, if you want your movies on your iPod or iPad, is a good format to have them in). Just fire it up, hit "open", navigate to your movie file, and MetaX will present you with metadata search results from the TagChimp database. You can either double click on the result that looks best to you, or check off just the tags you want to add from any given result. MetaX also supports a queue, letting you tag multiple movies at once, which is great if you have a rather large collection. It's pretty easy to use, and comes with a great set of instructions for those that would like a more advanced set of features (like fetching metadata by scanning bar codes with your webcam).
MetaX is a free download for Mac OS X. There is also a Windows version available here. Thanks, Norman!