In The Bag - A sporting event with a pre-determined outcome.
Inside Bet - A bet in roulette on a single number including 0 or 00, or any combination of numbers.
Inside Straight - In poker, four cards of a straight where the straight can only be completed one way.
Insurance - A side bet in Blackjack that's offered when the dealer's upcard is an ace. If the dealer has a Blackjack, the bet is paid 2-1. If the dealer does not, the insurance bet loses.
Insurance Bet - Making two (or more) bets at a craps table, one or the other of which is sure to win.