Online casinos have bonus codes that give you specific bonuses when you enter the code.

Usually you enter the online casino bonus code when you make a deposit and the bonus will entitle you to certain benefits, for example you might get a cash match on the amount you deposit or you might get a free spin of a slot machine.

Bonus Codes for Casinos
Slotsoasis ("4000FREE", $4000 free bonus offer)
Intercasino (changes monthly, click on promotions tab for the intercasino bonus code)
Smart Live Casino ("MAXIMUMBONUS" bonus code)
Betfred Casino ("MAXIMUMBONUS" bonus code)
Bet365Casino ("MAXIMUMBONUS" bonus code)

Online poker rooms also have bonus codes.  Here are some of the most popular:

Bonus Codes for Poker Rooms
Ladbrokes Poker  "MAXIMUMBONUS"  $1000 FREE
Ultimate Bet "MAXIMUMBONUS" $1,100 FREE
Absolute Poker  "MAXIMUMBONUS" $500 FREE
Full Tilt Poker "MAXIMUMBONUS" $600 FREE
Blue Square Poker "MAXIMUMBONUS" $500 FREE

Online Sportsbooks also have bonus codes. Here are some popular bonus codes:

Bonus Codes for Online Sportsbooks
Boylesports  "MAXIMUMBONUS" £200 Free
Blue Square "MAXIMUMBONUS" £50 FREE
Bet365 "MAXIMUMBONUS" £100 FREE
Stan James "MAXIMUMBONUS" £25 FREE