Within a mission, just type www.freespace2.com, and you should see a Cheats activated message. Then, hold down ~ (tilda) in combination with the following:

Shift-C - toggle availability of countermeasures for all ships
K - kill target
Shift-K - destroy targeted subsystem
I - invulnerability
Shift-I - toggle invulnerability for target
Shift-W - infinite weapons for all ships, including yours
W - infinite weapons for just your ship
G - mark all primary goals as completed
Shift-G - mark all secondary goals as completed
Alt-G - mark all bonus goals as completed