Blasphemous as it sounds, I really like Halo. I like the floaty jumps, the slugfest combat where landing the first hit doesn't always mean a win, the swooshy, slidey vehicles and the range of multiplayer modes. So naturally, Microsoft chose to release the free-to-play Halo Online in Russia only and cancel it before it could leave the early beta stages of development.
Legally fuzzy, perhaps, but thanks to a highly dedicated community and a lot of open-source poking around, development of Halo Online has quietly continued for years. Today, ElDewrito 0.6 is live, and it's the best PvP Halo experience you can find on PC today.
The current version of the mega-mod project known as ElDewrito includes everything present in the final version of Halo Online, plus a good chunk extra. The Forge - Halo's level/game-mode editor - is fully integrated, and you can host custom maps and modes on your own servers. Beyond that, more advanced modding is possible, although most of the several thousand active players are just playing the vanilla version of the game at present.
Halo Online is based on Saber Interactive's PC port of the Halo 3 engine, so the competitive core of that is present and correct, minus the (excellent) Firefight co-op/horde modes. Sadly, it's just human players only, but after a few minutes hopping between servers, I think this'll remain well populated for some time to come, especially if updates continue.
Update: The server browser woes mentioned below are now entirely fixed, after they rolled out a new and far more aesthetically pleasing browser screen that appears to have none of the issues present in the original release.
There are some rough edges in the current build - the server browser itself is a bit clunky and slow, and I found myself accidentally joining games by clicking a little too fast through the UI. Also, the Forge, while powerful, only lets you remix the existing dozen or so maps that Halo Online had when Microsoft pulled the plug. The expansive creators sandboxes aren't in the present version. While there is the possibility that Microsoft may bring the hammer down on ElDewrito at some point, it seems unlikely given how long the project has been active. Plus, Microsoft's newfound live-and-let-live attitude towards Halo fan-works.
You can grab the latest ElDewrito installer off the project's official site here, although if you want to poke through the source code and maybe lend a hand with the project, you can find the GitHub page for it here. Tangentially related, but if you want something a little more single-player oriented, you'd do well to check out Halo SPV3, which we previously covered here; A fan-made reimagining of the original game, integrating enemies and gameplay elements from as far into the series as Halo: Reach.