If you've ever needed to take a batch of files and rename them a certain way, I recommend using ReNamer, by den4b. As deduced from the generic name, it wasn't easy to search for so I could download it in the first place, but once I learned the mechanics of how to create rules, it was as easy as dragging-and-dropping folders with the files onto it and applying presets. Presets are doubly great for saving you time in the future, when you need to do the same sorts of processes to more files.
ReNamer's rules are extensive, and I haven't personally had a need for some of the more esoteric ones, but what's really nice is you can preview changes rapidly before committing them, saving you from scroo-ups. For example, here's a simple setup that renames "Snapshot_XXX.bmp" (Second Life's saved-to-disk pictures) to a more descriptive "Inworld Exploring – X.bmp", which I then uploaded to Flickr:
It sure beats going through by them one-by-one.
Better filenames definitely help with later searchability too, since I've been finding that even if I don't use keyword tags, sometimes putting important words in the title is good enough. Another project I've recently used ReNamer on is retitling my solo piano 2 recordings sequentially from the original files.
So if you've got series of photos from your digital camera that deserve descriptive filenames, want to uniformly tidy up a collection of text docs, or another usage you can imagine, I suggest giving ReNamer a go.
One more reason to smile: ReNamer's free!
{ 4 comments… read them below or add one }
I used to use Total Commander (at http://www.ghisler.com/) for renaming multiple files at once. And besides that, did everything else file related with it too, from zipping stuff to viewing to starting programs to ftping. Just a tip
Why I don't use it anymore? Because it's a win app and I've switched to Mac 
In the same vein, there's a program called File Renamer that does some similar things and has proven very useful to me. Since I haven't tried the other programs mentioned here, I'm not sure how similar/different they are, but more options are always better, neh?
@Daedalus and @DarkLightEternal: Thanks for those recommendations, always good to have more to choose from, as long as you can make a decision and find something that works well for you.
I had a look at File Renamer before but personally found it too unwieldy and cluttery. I like ReNamer's straightforward (after you understand how the rules work) approach. 
I just use foobar2000
I know it's an audio player, but the scripting capabilities it has are superb.