Post games (and other similar forms of amusement) that you want to recommend to other Anons. I will start.
Halo Custom Edition (Halo CE) is a official expansion for the PC version of Halo: Combat Evolved (Halo PC). What makes Halo CE different from Halo PC is that players can create custom levels (and weapons, etc.) for the game. Pls note that you need Halo PC product key to install it (but you can just search YouTube for Halo PC keygen and copy the key from the video). You don't need to have the CD, if you install the patch.
You can get Halo CE here: http://hce.halomaps.org/index.cfm?fid=410
You also need the official V1.0.10 patch: http://hce.halomaps.org/index.cfm?fid=6798
You will probably also want to install the Halo Anti-Cheat (HAC2). It's not required but it makes your life easier. It enables things like automatic map downloads when joining a server. You can download it here: http://client.haloanticheat.com/release.zip
To install it, just copy the "loader.dll" to "C:\Program Files (x86)\Microsoft Games\Halo Custom Edition\controls" folder.
If you want to create custom levels and other content, you will also need the Halo Editing Kit (HEK): http://hce.halomaps.org/index.cfm?fid=411
AND the bug-free tags: http://hce.halomaps.org/index.cfm?fid=6690
To play custom single player maps, create a new shortcut for "haloce.exe" and add "-devmode -console" as launch options (without quotes). (I think the Halo CE installer adds Halo Custom Edition Developer Mode in your start menu by default.) Use that shortcut to launch the game. Then you need to open the console (press the key under ESC), and type "map_name map_name_goes_here" (without quotes and the file extension) and press ENTER.