A buddy of mine runs a weekly gaming session, and the players all have pretty different and fluctuating schedules, which makes it pretty difficult to get everyone together at the same time.
Instead of waiting for a time when we can all get together, we often go ahead and play without one of the players. This, of course, raises the problem of what to do with the missing characters in-game. Rather than trying to contrive some reason why the druid keeps vanishing at certain points in the game, we simply adopted the humorous explanation that party members just randomly turn into kumquats, and be done with the charade of trying to justify the missing characters in-game. It adds a little bit of fun as we decide who will be carrying the "Kumquat of Druid Summoning."
The other problem playing without the whole party is that the absent player(s) will not know what happened since the last adventure, so you end up spending valuable game time rehashing what has already happened. (Or worse, you leave out some important detail that is critical later on, which is easy to do when it's been one or two weeks since you played last.) To solve this problem the DM just set up a blog called "Never pick up a duck in a dungeon..." which simply chronicles each night's adventures with all relevant details. Missing players can just read up on what happened before the next game session to get caught up, and it will serve as a fun adventure log to read later.
The blog only has one night's worth of play so far, so it's probably not a great read now, but the idea is something that I think would be a worthwhile addition to many game groups.