The promised SDK update from Valve is finally here.
Now those of you clever enough to understand difficult things can get to creating your own Portal levels, including maps, models and textures. Which means there's no excuse. It also includes all you need for mucking about with Episode Two and Team Fortress 2 content.
If you're new to the Source SDK and want to get your hands on it, you'll need to already own a Source-based Valve game, and the open Steam, go to the Tools tab, and install Source SDK. Then head to the Valve Developer Community Wiki where there are detailed notes to get you started.