The Showdown Calculator included in Poki's Poker Academy pretty much does exactly this. You can select your starting hand, the number of opponents, the cards your opponents have (or just let them have random hands), any of the cards on the board (or just leave them at random cards). You can then tell it to run anywhere from 100,000 to 5,000,000 simulations with that setup and it will tell you what percentage of the time your hand will win, tie, and lose.