You can make your own levels on the pc. It is a bit of a painful process currently because Valve hasn't released their official tool yet, but they said they will release one sometime after launch :y:
If you just really want to get your hands on it, here is a tutorial. Probably best to wait for the actual tool though :p
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Custom maps will be able to be accessed on on the pc version and the ps3 through the steam feature from what I have read.
Valve has also said it will be bringing the best user created levels to all consoles through DLC. Knowing Valve it will likely be free on pc and ps3, who knows about M$
Source for that ^
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TLDR version = Its sorta like LBP, and it will be even better as time goes on =D