http://www.exitemod.com/

Portal like multiplayer goodness for hl2, without the episode 2.

Few more details at http://www.exitemod.com/forums/index.php?showtopic=123

Been playing it all afternoon, plenty of bugs if you try to get smart with the portals, but well worth the download.

By far the best portal mod I’ve seen in the source engine so far.
There is a portal mod for UT2004, that is bug free and work great.
Grab it here: http://utforums.epicgames.com/showthread.php?t=530848