I did a "mix" of several songs a while ago and used Switch (Download button at top-"navbar") To convert the stuff to .WAV (I have found few programs which works with .MP3 directly)
Then you can use for example Kristal, which I find rather handy, with fades, overlapping, etc. just be sure to consult the manual, can save you hours by quick-commands
Both these are freeware (i.e. the non-plus version of Switch) and I found them quite useful.
If you just want to stuff the MP3-files together in a row, without "trimming the edges", I think there could be easier solutions, though don't know of any...
btw, shouldn't this go under music or something?
- Arand