A good overview can be found at
The Texas Moratorium Network.
Of the 9 Democratic candidates currently in the field, 3 oppose the death penalty in all circumstances. They are Dennis Kucinich, Carol Moseley Braun and Al Sharpton. Kucinich and Sharpton always have opposed the death penalty during their public careers. Kucinich has just introduced a bill to abolish the federal death penalty. Moseley Braun introduced moratorium bills and voted against reinstating the death penalty in Illinois as a state representative. Later, during her successful 1992 run for U.S. Senate, she briefly endorsed the death penalty, and was attacked by her Republican opponent for her inconsistency. Following that attack, she resumed her opposition and has been consistent and outspoken ever since (see Chicago Tribune article below).
More here.
This site has some good detail on some of the candidates whose positions are unclear. It also has good resources and links, and even a poll on the front page, so it's one for your bookmarks. Current poll results:
6% - Carol Moseley Braun (3 votes)
22% - Howard Dean (11 votes)
2% - John Edwards (1 vote)
2% - Bob Graham (1 vote)
10% - John Kerry (5 votes)
48% - Dennis Kucinich (24 votes)
2% - Joe Lieberman (1 vote)
8% - Al Sharpton (4 votes)