Maybe look at Wikipedia's list:
Prospective candidates
The following are individuals who are or have been the subject of speculation in prominent media sources as being possible presidential contenders in the 2012 election. The speculation may stem from noted media analysts and commentators, or from actions or comments made by the individuals themselves — which suggest the possibility of a presidential run — as reported in reliable media sources.
Democratic
President Barack Obama is eligible for a second term.
Other prospective candidates:
Former Senator Mike Gravel of Alaska
Former Representative Alan Grayson of Florida
Former U.S. Senate nominee Alvin Greene of South Carolina
Gravel is 80 years old, Grayson is a buffoon, and Greene is just ignorant.