Bring-up the console by pressing Tab+Enter.
Type the word 'PASSWORD' and when the console asks for the password type 'ROCK'. This will enable cheatmode. Now, exit the console and return to the game.
When the cheat mode is activated, pressing the following keys will result to the correspoding cheats:
F1: add lives
F2: add specials
F5: turns-on god-mode (invurnerability) (turn-off with Tab+F5)
+: moves the power-up selecton indicator up
-: moves the power-up selection indicator down
For those who are not familiar with the game, Space Hog is a 1 to 2 player, space-shooter game created by freelunchdesign. The game can be downloaded for free from their website.