Loek Jehee has released a free AppleScript Studio project which enables the easy combining (or splitting) of files. Typically, you'll run into multiple-part files if you've downloaded a large binary file from a newsgroup. Using Split&Concat, you can easily recombine the parts into one complete file.
Loek's looking for beta testers to help him find any possible bugs and improve the product; if you're interested in helping, there's an email address on his web page.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020103220419942