While we wait until this April 28th for Valve's official announcement on the Half-Life sequel, a few details have been leaked out already as well as some screenshots (mostly magazine scans). I took the liberty of copying what one of our forum members posted in this HL 2 thread in the forums, thanks Svoboda...

These are the details known so far, can't wait for those high-res shots...
» Half-Life 2 is scheduled for a September 30th release.
» Yes, you're still Gordon Freeman, and yes, the story assumes you went to work for the G-Man.
» Half-Life 2 takes place in a European city called "City 17."
» Headcrabs will be back! Although some of the Xen aliens will be your friends?
» Barney is back. And so is Half-Life's original writer, Marc Laidlaw.
» While the new engine has all sorts of fancy features, it's still designed to work on lower-end machines. Apparently a 700mhz processor and a video card capable of running DX6 is enough, although a 2ghz with a GeForce4 is recommended.
» The girl in the promo art is named Alyx, a daughter of a scientist who factors into the story somehow. She's not playable.
» Scripted Sequences will be more interactive.
» Half-Life 3 is planned, so expect another cliffhanger ending.

HALF LIFE 3 WOOOOOT!! HELL YES!!! hehehehe

The screenshots they were scanned looked pretty interesting, particularly that massive walker droid it showed, very War Of The Worlds like.
Would be nice to have some Duke Nukem Forever info. get posted too though

You know what, the hell with Duke Nukem Forever. I am sick of hearing about it and IF it does come out I am not buying it, nor any 3D Realms games. They might as well ditch that project. There are much better games coming out and DNF is the last one on my mind right now. Max Payne wasn't a bad game and I played through it twice but they need to get off their butts and do something productive. 3D Realms quit smoking the cheeba and do some real work! They even abandoned PREY. Pissed me off because I was so looking forward to Prey coming out but they ditched the project and then all of the sudden all PREY related material on the web disappears like it never existed. Hell with'em!