I am at the level after east black mesa. I am taking my time though.
I have found about 10 cool things, woulda missed them going fast.
CHEAT CHEAT CHEAT CHEAT
The best one so far.
In the last of teh water works, in teh area where you have to take out the helecopter. Open the gate and go to the east, there are two drainage tunnels, one gate is slightly open. If you get a barrel or ues you prop boat you can get in. Walk through the radioactive sludge in the tunnel and almost at the end you can turn to your left. Talk to him and see whats on the fire mmmmmmm. You also get health and ammo.
Or.
Grab a board with the gravity gun and angle it up aginst the wall, if you do it right you can run up the board and jump to ledges before it falls.
Or..
Use the gravity gun to pick up the saw blades and chop up you victims by firing it.
Or...
In the three room water trap, where you have to raise the water level to escape. There is a duct in the first room you can reach by climbing a ladder. Grab a barrel and take it to the far end of the room where there is a vent, put the barrel on teh duct and jump on top and duck. Staying ducked down and jumping forward you can climb through the vent and get life and ammo. There is also a room above, I don't know how to get into it though.