When playing video games, I often prefer games that are easy to drop in and drop out of. Games like Team Fortress 2 where players are always respawning so the new player isn't set back, especially if most people are ignoring the goal.
With board games I sometimes see two problems: Sometimes the game stretches on longer than one player can afford or care for (I usually have a 1 hour rule for myself). Sometimes the game has already been started and another prospective player shows up.
In these circumstances it would be nice if a game carried on even though players might start or stop playing at any time. Are there any board games out there with good drop-in drop-out mechanics?