The 64 Bar Challenge mix I did, 29 tracks in 52 minutes, the brief was for each track to be 110 beats per minute and last 64 bars. The series was originated on the Ninja Tune forum by Kovas and driven over 6 different challenges with entries from all over the world.
The quality was really high, largely leaning towards the electronic end of the spectrum, and with roughly 50/50 of the entrants being unsigned or never having released anything. Take a listen and see what you think and you can pledge for a vinyl version of the tracks on a compilation over on Beat Delete.