I also use Audacity which is freeware, but never to blend songs together into a set. Audacity is more of a general wav editor. It could probably do it though: you'd 'import audio' to add a stereo track for each song and then tinker with the alignments. There are definitely other audio tools for DJ-type work that would be more apt for that purpose
I've created my own tracks in Audacity, and I've also used it to dub old cassettes and stuff. But lately I've been buying mp3s off Amazon and I find that I almost always need to load the mp3s into Audacity to raise the level or delete extra dead air at the end
