Put these commands in the console, the console as you should know by now is accesible during game or before, bind it in the controls to a key, or by default its mostly the ` key (top left key under escape


developers 1 - shows you all sorts of informations on screen, such as r speeds.
r_speeds 1 - shows you the r_speeds on screen (r_speeds are w_polys, e_polys will go up the more complex your shapes are in those areas.)
cl_showfps 1 - shows fps on screen
net_graph 1 - shows fps and choke etc.
sv_gravity 100 etc - jump to places you cant normally get to

halflife single player only:

sv_cheats 1
god 1
noclip 1
notarget 1
noclip

Handy multiplayer one

hud_saytext_time 0 - turns off chat when spammer / idiotic person on server is annoying everyone.

There is also a gl_wireframe 2 but this only works in single player to stop people using it to cheat in multiplayer.(this command is good when your trying to figure out an r_speed issue,cause you can see what the engine is drawing while looking in a direction)(it's also good to see how your textures are being applied on a brush and how the engine resizes the textures on a brush).

 

*UPDATE* Slacky.............

So if you need to get into gl_wireframe 2 mode just open map thru console with "deathmatch 0" or compile the map with deathmatch left out of the command line.(Just make sure you add it back in when you do a final compile)