Mixxx is a powerful open-source DJ tool that gives you everything you need to create quality live mixes.
If you're a total DJ newbie who knows nothing about "key lock" or "hot cues" then you can get started with the Auto DJ: just import a few audio tracks, and the program will handle the mixing from one to the other.
If you're a little more experienced then you'll appreciate the BPM detection and estimation, which helps you beat match songs and cross fade them for a perfect mix.
And if it turns out to be not so perfect after all, then nudging mismatched tunes can be as simple as a click on the program's new ramping pitchbend buttons.
More advanced features include good MIDI controller support, vinyl control, and multichannel soundcard support (both playback and capture). And features like hardware video acceleration help keep latency to a minimum and ensure you're always working with the most responsive of consoles.
See the release notes for 2.2.3 information.
Reasonably straightforward to use and with a good selection of features, Mixxx is a great place to begin your DJ career