Depends on the campaign. For my last few...
3.5e Eberron game: XP by the book. All PCs get an equal share, whether the player could make it or not.
WFRP: 100 XP (1 advance) per "half session". Due to the group size, we cancelled if anyone couldn't make it, so no issue with absentee players.
SWSE: No fixed XP. Characters gained a level every 3 "half sessions". As above, there was no issue with absentees.
Recent 4e games: No XP tracking. The DMs said, "level up" after ~10 encounters. I have no idea what they did with absent players - the last session I was able to attend was the one right before everyone levelled up!
I'm currently working on a system where PCs "buy XP", as discussed in a recent thread on the topic ("An Older Experience System", or something like that). I expect to roll that out with my next 3e(-ish) game.