Players in absentia...

[CyCo]

These days I just have someone else in the group run the character, and give them XPs as normal. Those at the table get first dibs on any gear/treasures however.

I used to only give 1/2 the XP to those who didn't turn up but gave notice, and no XP if someone didn't turn up and didn't bother to tell anybody. That only ever happened once or twice, the everyone got the idea to at least notify me that they
couldn't make it.

Had a few complaints about the lower XP, but it didn't really matter that much, as back then my group numbered around 8 or 9 a session. These days my group is smaller 4 (plus myself), so if someone starts falling back, it's more noticeable. Also, no complaints.

[Misfit KotLD]

We always used the Friendly Body Bag of Holding. The missing player's PC goes in the bag and comes out when the player returns. Nothing bad happens while in the bag, but nor does anything good.

[Noon]

What complaints?

If the game is fun, it can probably stand some minor complaints happening and still run fine.

[Ice Dragon]

Depending on the setting/game/O.C.C. I find a way to remove that character from the game session.

e.g. the mage of the group can study in a town to learn a new spell (he still has to pay )
e.g. the power armor pilot has a mechanical break down which he has to repair before entering any form of combat
e.g. the officer is recall to fleet command for a briefing (next adventure ...)

and so on.

The character will not earn any XP or gain any treasures (except his/her/its team mates share something ).

[Noon]

Make a back up adventure for if that player isn't there.

But really, if your deciding a session is based on one player, your probably deciding more of the story in advance than you actually want to.