This past year cemented Twitter as a service that has definitely changed the way we think about communication. I enjoyed the following retrospectives about the best (and worst) tweets of 2011:
Mary Elizabeth Williams at Salon has a nice overall list of the best and worst tweets of the year.
Time magazine also has their own best and worst tweets list, most of them with a political slant.
The Wall Street Journal has a list of the best celebrity tweets of 2011.
Warming Glow has compiled some pretty amazing tidbits from the world of television.
Mashable has a list of 2011's shocking social media disasters.
[And these are not really tweets, but New York magazine has some of the best quotes of the year]