I thought I'd share over a few articles some of my more system-generic house rules (remember the Order of the d30?). Perhaps the most enduring (featured in nearly all our games) are Mulligan Stones.
From our house rule document:
"Mulligan Stones will again be used. They may be used to re-roll any player dice roll. Every character will start the game with one. They are handed out (sparingly) for good roleplaying, teamwork, great one-liners, fervent game participation, and general awesomeness. In a new twist, they may be used to add +5 to any one player roll (limit one Mulligan Stone per player roll).
A player may use their Mulligan Stone on another player's roll, provided that other player agrees".
This sort of rule appears in many games by many names--Action Points, Cinema Points, Bennies--but it all boils down to the same thing. This is just our take on it, and since I generally run fairly demanding games, Mulligan Stones have become highly prized in our group.