These are some of the things I've created in the Hammer World Editor. The screenshots here are only a small portion of what I've done with the program, since a lot of it isn't aesthetically pleasing (Which include most of the stuff I've made for Zombie Panic: Source in my earlier days of mapping). I'm still learning the features offered by the Source engine, so the things that I currently create may be a little rough.
This was one of the first maps that I actually released. It's for Half Life 2: Deathmatch. Some custom textures were used.
When I wanted to run a home-hosted Garry's Mod server for friends and I to mess around on, I wanted a few specific features in a map and couldn't really find them. I quickly threw something together in an afternoon which did. It's nothing too fancy, it has a grassy area for building, a round section of water for boat building and some houses for some reason I can't quite remember. The filesize isn't too large, which was important since I was hosting the map on my home internet connection.
I called this one "Substation". I got fairly far with this one, though shelved it due to running out of ideas.
Here's a video of that spinning room in the above screenshot:
This is just a building I created on a day off from school in the Episode 2 version of the engine.
I've mostly stopped creating maps unless there's something specific I want to do in say, Garry's Mod.