If you are on the look out for something quick that has worked elsewhere though I hope that this small list may help you save some time.
First up is a collection of games that I have known for some time although have not tried all of them.
the Retro Wiki http://retrospectivewiki.org/
Next up is a format which is not on the Retro wiki and I have done in several enironments and I have also recommended to a friend who has done it and the feedback was good.
The cool wall Retro
Tasty cupcakes was recommended to me although I was not able to find any games suitable for retros on there. Perhaps some can be adapted or activities used in other types of games could be borrowed and used in a retro. Useful site to have in the bookmarks anyway.
Multi team retrospective - this is a format I have used in two variations in different companies. Works fairly well and I'd recommend it when you have multiple teams doing very short (e.g. 1 week) iterations when it is important to keep the length short. I explain it here.
How We Do a Retrospective - Tim Wingfield explains how his team does retrospective. Looks a good approach to try, certainly slightly different combination of ideas compared to what I've seen before.
[Added in Sept 2011]
Furturespective (also Future-spective) is a way of driving possible improvement opportunities by shifting your focus into the future and then looking back from there. It is described well by Anders Laestadius here and also mentioned by PierG here however it is not exactly the way we do it in my current project. So I decided to write up a summary of the "Furturespective the way we do it" here.