Here are some simple ways you can bind functions to keys. First, press your talk key (default is ~). Delete SAY. Note that you'll have to re-type in SAY and hit enter to exit out of the chat. Then type the word BIND, followed by a space, followed by the key you want to bind, followed by a space, followed by the function. Then hit Enter. Here are some examples:

BIND X SAY Nice shot!
BIND Z SAY_SPECIES Take down my killer, he's low on health!
BIND C CROUCHMODE 1
BIND V CROUCHMODE 0
BIND O QUICKSAVE
BIND P QUICKLOAD

SAY is obviously the talk to everyone command. SAY_SPECIES will only appear to players of your same species, useful in team deathmatch. CROUCHMODE 1 changes the alien's crouch mode to toggle rather than having to hold it down. CROUCHMODE 0 changes the alien's crouch mode to holding down rather than toggle. QUICKSAVE lets you save the game and QUICKLOAD lets you load the game. You can also bind keys to cheats if you're in debug mode.

Once you've bound a key in game, it will remain bound even after you quit and restart. You can easily unbind the key, too. To do so, delete SAY, type in UNBIND, followed by a space, followed by the key you wish to unbind, then hit Enter. Here is what it would look like:

UNBIND X

If anyone knows any other bindings, please list them here!

Very clever, thanks!

You can just to "listbind" to see all the keybindings.