FiRE is the File Renaming Engine, a powerful tool for the batch renaming of files.
Is your music collection a little messy, for instance? Point the program at the folder containing whatever files you'd like to rename, click File > New > Metadata Rule, and begin selecting the tags you'd like to form the name.
Track (the track number), then "Plain text" (a separator) and "Title" (the track title) might work, for instance, though there are plenty of other options ("Album", "Artist", "Comment", "Composer" and more). The main window shows you the effect your changes will have on the file names, and when you're happy, clicking "Apply Changes" will rename the files for you.
This would be useful enough on its own, but it's actually just the start. If you don't want to (or can't) use metadata, then you can add a "match/ replace" rule instead, which can replace one phrase with another, and apply consistent capitalisation. (There's even support for regular expressions if you want to get particularly geeky.)
And the List rule can turn files into a list, adding regular numbers (1,2,3), Roman numerals (i, ii, iii), or letters (a,b,c) to the front of each file name.
If there's a small problem here it's that the Eclipse-based UI means the program is larger than you might expect at more than 20MB (although FiRE is also portable, so there's nothing to install: if you don't like it, delete the download and you're done).
But on balance it's an excellent renaming tool, versatile and packed with essential functionality.