Depends on the hangover. If I'm on spirits then it's a case of having a very sore head and feeling like death warmed up.
Turrican fits the bill - with the sound turned down of course. If I'm in a really bad way then its Doom.
If I'm on beer then I'll be dehydrated and sluggish, so a decent flight sim is my choice. Either that or it'll be something like Civilization or Warlords because I get so absorbed in them I forget about my fragile state of health.
