By donation, this is the way upon which Wikipedia rely, which is, IMHO, the most ethical economic model.
We already ticked that out, because it only works with really big FOSS projects - let alone game projects. (read thread)
As shareware with a lifetime-license fee which covers and allows to upgrade the game installed on proprietary operating system, and as FOSS game, free of charge, for the free open source operating system. (This is the economic model for Xchat)
Won't really work (there's the silverex.org build, you see). (read thread)
By sponsoring, if your game represents some common values with an organization or a company.
Highly unlikely, however yet a concept that can be explored
(FOSS WWF/Greenpeace/PETA games? heh)
By subscription, this economic model fits perfectly for MMO but I don't think it will be transposable for other type of games.
Again, read the thread for that.
By targeting several platforms, you should consider to sell the game on WiiWare or AppStore or other software retailers, even if it involves to change the license of your game for this platforms.
Read thread.
Related note: There is a survey that tries to gather some information on what people would most likely demand from a FOSS-driven/Linux-oriented game development company - obviously in a hypothetical case. Check
this thread on ubuntuforums.org for that. Results ought to get posted after the holidays.