What cool mods are out there? am thing like SPV3, Operation Trebuchet and Sins of the Prophets, can anyone tell me about another
I'd imagine you've heard about Installation 01 if you've heard about those others. Basically it's an all-multiplayer fan game in the style of Halo 3 per gameplay but with updated graphics and a couple of new maps. Things include Playable Elites, Dual Wielding, and other stuff. It isn't out yet but so far what the devs do have shows promise. I'd also implore you to check out Project Contingency, made by the same guys working on I01.
There is this particularly cool Left 4 Dead 2 mod which changes all the zombies to various flood forms, all the weapons to UNSC equivalents and the characters to ODSTs. While the mod is only visuals, it looks quite cool.
Project Lumoria is the best vanilla CE experience, its a short campaign with a few new enemies and vehicles added, and doesn't require Open Sauce or the HEK. It's really strange to play, it kind of feels like the child of Halo CE and Halo 3 in the style and architecture, but the music is almost all from Halo 2. The voice acting is predictably mediocre, but its a mod, so you can forgive that. There are also some things added that were cut from the original, like elites with jackal style arm shields, and they also add the Fuel Rod Cannon and the Flame Thrower as they appeared in the Custom Edition multiplayer. And they have terminals. All up its a really well designed short campaign, for if your craving more of the CE campaign but bigger.