Gearbox's impressive PC port, Halo PC with Custom Edition makes this an addictive game.
Halo PC fixes this with online multiplayer and to top off that, you'll get Custom Edition for an optional free download which includes tools needed for map making.
User-made maps can be downloaded off various websites. The most popular of these is halomaps.org. The custom content is among the things that extend the time you spend with the game and maps are still being made as we speak. Mods like Project Lumoria, CMT SPV3, some more campaign maps, a slew of Firefight maps and many more are just a taste of the game's vast user-made content.
For a game this old, the community is still very much alive. It's amazing to see how Gearbox's Halo PC port lasted this long.