From Locus Online:
Locus Online is hosting, during the month of November 2012, a poll for the best novels and short fiction of the 20th and 21st centuries, the first such poll Locus has conducted since those hosted by the Magazine in 1975, 1987, and 1998 (with an online supplemental poll in 1999). The scope for this poll is the 20th century, 1901 to 2000, and the first decade of the 21st century, 2001 to 2010.
There are five categories in each century: SF novel, fantasy novel, novella, novelette, and short story. For 20th century categories, you may vote for up to 10 items in each category; for 21st century categories, you have the usual 5 items in each. Results will be scored based on rank, so that a 1st place vote is worth twice as much as a 5th or 10th place vote, but not 5 times or 10 times as much.
The ballot is only live through November 30th. Check it out here.