Mapping folk are never particularly slow about churning out maps for new games, but the rate at which Portal stuff is appearing nevertheless surprises me. A whole bunch of new maps can be found over at Portal Maps, varying from the absolutely basic sandbox to the fairly complex challenges. I've only had a chance to play through a couple, but some, such as Glasschamber, seem even more fiendish than the Portal levels themselves.
Meanwhile a search of Moddb reveals that no Portal modding projects have yet been announced. Tsk, get a move on Modders. (Yes, that's your cue to link to a Portal mod, readers.) I guess in the meantime we'll just have to play through all of Half-Life 2 with the portal gun.