Leftist science fiction

Mack Reynolds. Reynolds' novels feature "hyper-industrialism", where most of the necessities of life are provided by highly automated factories and most people do not perform productive labor (they're supported monetarily by the Negative Income Tax). He also features a professional-managerial "meritocratic" ruling class. Sadly, most of his work is out of print, but it can still be found in used bookstores.

Notable works (that I've read) include Commune 2000 A.D. (1974) and The Towers of Utopia (1975).

The works of Greg Egan. Many of Egan's works — especially Diaspora (1997); Schild's Ladder (2002), and Incandescence (2008) — describe a post-industrial "virtual" society, where consciousness is implemented both biologically and electronically. Productive activity is entirely voluntary and ethical beliefs have widespread agreement and near-uniform compliance. Although perhaps Utopian, these works are probably the best fictional description of a classless society. He also discusses a more contemporary vision of a politically isolated left-anarchist society in Distress (1995). He seems extremely liberal/progressive, atheistic and not particularly religion-friendly (see especially his novellas Oceanic and The Moral Virologist). In addition to political philosophy, his novels and short stories explore subtle philosophical issues of ontology, epistemology and ethics.

Gene Roddenberry's Star Trek, especially the original series, is often considered socialist, depicting a society without money. Later series use the often slightly ridiculous alien Ferenghi to poke fun at modern capitalism.

If anyone has any suggestions, please offer them here in comments. I think science/speculative fiction is an excellent medium for discussing and exploring political philosophy and philosophy in general.

Update: China Miéville's Fifty Fantasy & Science Fiction Works That Socialists Should Read (thanks to a commenter)