If you've never tried out Garry's Mod
for creating Half-Life 2
content, then you're missing out on a cool little sandbox program to have fun with the Source engine. With the great success of the mod, it's being moved to a commericial product and being sold
through Steam. With the change should come a more robust product as Garry will get the full source code now to dabble with. The pricing on the product will be $10 so it'll be interesting to see if people will pay for a product that's been free for a while now.