Hidden room in the OpFor boot camp. It has the developer's initials (DMM) and 1999 in it. The elevator in "Forget About Freeman" was apparently part of the map that didn't make it. The button changelevels into the Lambda Core level into the vehicle bay with the tank. (You die, though). If anyone can't find the hidden room, ill post a pic in a while.
You can also check out your buddies and check out the Osprey that you fly into BM on in Boot Camp, apparently when the sergeant says "now go on your mission" or whatever, you would have to run to the osprey. Removed from the final release.
And I got lost in the biodome, it was too confusing.
And the Bshift outro was cool, the car was neato, and I like the sci hacking away at the fence with the crowbar. The "ba_maint" level was also cool, with the sci messing with the console and getting shocked, and barney leaping up from his chair.