The year's best science fiction - Twentieth Annual Collection by Gardner Dozois and Ian R. MacLeod and Ian McDonald and John Kessel and Gregory Benford and Geoff Ryman and Steven Popkes and Richard Wadholm and Alexander C. Irvine and Greg Egan and Michael Swanwick and Nancy Kress and James Van Pelt and Eleanor Arnason and John Meaney and Kage Baker and Walter Jon Williams and Alastair Reynolds and Paul McAuley and C.C. Finlay and Molly Gloss and Robert Reed and Maureen F. McHugh and Chris Beckett and Charles Stross and Bruce Sterling