Chroma, by Harmonix, build 1532151
An unreleased Music-based FPS, formerly under NDA'd Closed Alpha, now cancelled (and NDA lifted).

The game itself mixed FPS systems with some rhythmn mechanics. For instance, you can jump at any time, but if you jump on the off-beat you jump considerably higher. You also have a dash, but that only works on the beat. Each team has their own soundtrack style (Electronic or Rock), and each weapon class adds a different instrument to the mix. If you reload, you get a rhythmn track, and if you match the beat you reload faster and get extra shots. Engineers have dual pistols that fire on a guitar hero like rhythmn track, and the three levels of deployables have longer and more complicated patterns you have to match.

While it may sound complicated, it was quite easy to pick up, and very rewarding to master the mechanics.

This is the complete client package, which will play both tutorial levels from the in-game menu, and (with some command-line poking) host and join a multiplayer server. However, the server implementation in these client files is sadly incomplete, so not all aspects of the maps work.

To start the base game, run "Chroma.exe" in "binaries/win32".
To host a server, add the following command-line arguments:

"Chroma.exe cp_darkwaters.udk?class=ChromaGame.ChromaCPGame?listen=true?bIsLanMatch=true"

cp_darkwaters.udk is the name of one of the maps, the others are:

cp_chroma_training1.upk
cp_clubchroma.upk
cp_locus_koth.upk
cp_packoffive.upk
cp_satellite.upk
cp_studio_koth.upk
cp_weapons_tutorial.upk
tdm_changeup3.upk
tdm_ghosts.upk

It's built using Unreal Engine 3, so anyone with familiarity with that should be able to get things going a little better.

There are videos up on YouTube showing gameplay (including one from myself)