Halo 3 (X360)
Halo 3's Forge isn't so much a level creator as it is more of a level editor. You can place crates, turrets, weapon, shield, and spawn locations, build walls out of boxes to block passages to change the flow of an entire map, and so on. Now that may seem lame to some, but there are some really cool things you can try out that show that some people (not me) can think outside of the box!
I'm sorry that I couldn't find some official tutorials because apparently a prerequisite for posting a user-created game-related video is to have really shitty ass rock accompanying it. It's like adding a motorcycle to Bill Gates-- it doesn't make him look any less of a geek. You're a hardcore Halo fan-- be proud of it! Anyway, these user-made tutorials show the possibilities of making very cool set-ups and tricks that perhaps you weren't expecting.
Direct Link 2
If just altering levels by placing various objects isn't your thing, then Far Cry 2 is probably for you. I won't even bother explaining what you can do because it would not do the game any justice! Check out the video below, and prepare to be amazed.
Series (PS2, GCN, XBX, PC)
One could spend countless hours manufacturing levels to blast human opponents and AI alike in. A treasure trove of parts, options, settings, lighting, bot and weapon placement, vantage points, music, architecture, and much more were given access to the player to tinker and toy with. I can't express or even fathom how many hours-- days even-- that were lost through designing levels and fragging friends. Don't think it's nostalgia either-- this level creator and this series are damn good.
Super Smash Bros. Brawl (Wii)
Brawl's level creator allowed you to try out some cool stuff. You couldn't create levels as sophisticated as what was already developed, of course, but you could create some very hectic and very entertaining levels. Regardless, you were still limited to rather basic levels using only three themes and an assortment of scenery and objects. Unlike everything else in this article, the pictures of Brawl stages are actually my creations.
Build, Trade & Destroy (Wii)
Let's go from a big-budget Wii game with an okay level creator to a low-budget Wii game with an incredible level creator of this side-scrolling shmup. Almost anything you can think of is possible with the level creator. You can build a background using the scenery and buildings already including with the game or create your own. Maybe you like sprites? You can actually create backgrounds, enemies, and ships that emulate your favorite games or go completely fresh with your own ideas. Perhaps pictures will speak louder than words.
As a huge Mega Man fan since my childhood, when I read that an upcoming feature would allow players to completely create their own levels, I needed this game. This was actually the game I bought a PSP for-- just for that feature, and boy, did I get a lot of mileage out of it. You could easily recreate levels from the eight robot masters or start fresh using a tileset. Add enemies, items, powerups, spikes, hazards such as moving platforms, bomb blocks, the boss door, and much more to make your levels as simple or as maniacal as you desire.
Advance Wars Series (GBA, DS)
If the over 100 maps in each game wasn't enough content for you to enjoy, you could design your own battle maps as well. The only catch is that things like omega cannons and other mission-specific items were off-limits. Everything else? Fair game. Make a map as fair or as lopsided as you want. The most recent Advance Wars, Days of Ruin for the DS, allows players to send their maps to other players via Wi-Fi. Plus you no longer had to deal with only three save spots for custom maps. Now that's progress!
Mario vs. Donkey Kong 2:
March of the Minis (DS)
Had a level creator not have been included in Mario vs. Donkey Kong 2, I probably would have regretted purchasing it. However, that's not the case as I got nearly limitless enjoyment out of creating my own clever and not-so-clever puzzles. The creator is so fleshed out, like Powered Up, that you could basically see that the developers designed the single-player campaign's levels (save for the boss levels) entirely from this nifty little mode. Also like Powered Up, you could share levels with anyone around the world.