What is a useful program for creating a digital mix tape?

I’ve collected a mass of good hip-hop tracks, and I would like to create a 20 track mix to share some of my findings with friends.

I have been wondering if it would be best to make a non-stop mix wherein each song crossfades into the next, or if I should keep the tracks separate. I have heard mixes where even though the songs are separate files, the audio files still fade into one another: the next song starts to play at the end of the previous file.

Anyway, however I choose to do this, what is a good software to create such a thing in?

One last thing: I currently use Mac OS X 10.5.6 primarily, but I have Windows XP too.