SoX
http://sox.sourceforge.net/ - Super handy for merging, sample rate, and bit reduction. It can also take care of fades and has a ton of features I never used before. Available free for Mac, Win, and Linux.
I like to merge and re-sample on the fly, in one step, so as to not get funny results at the end points between files. Sometimes, the merged 24 bit files can be too big to work with so this program has been a savior for me.
This is a command line tool only so if you are big on GUIs you probably won't like it. The documentation is good though and it only takes a little practice to get nice results.