He wrote. He wrote really, really, really well.
In no particular order:
He wrote 2 Star Trek novels, How Much for Just the Planet and The Final Reflection each of which prompeted a New Rule (Don't Ever Do That Again).
He wrote various gaming products, including GURPS Time Travel and The Yellow Clearance Black Box Blues for Paranoia.
He wrote a few other novels, including Princes of the Air, the World Fantasy Award winning The Dragon Waiting, Web of Angels, Scholars of the Night, and the kick ass Growing Up Weightless.
He wrote short stories, including several Liavek stories, "Erase/Record/Play", and "Dateline: Colonus", which my Lit Geek self found jaw-droppingly cool.
He wrote poems. The one I suspect is most known is "110 Stories" about the September 11 and the fall of the towers. My favorite, and the first one I encountered, was "Winter Solstice, Camelot Station" for which Jane Yolen made Parke Godwin find room in the anthology Invitation to Camelot.
He wrote really, really, really well. He wrote in lots of different genres, making it hard to pinhole him, and, to a degree, to market his stuff.
He had enough wrong with him that it's amazing he lived as long as he did.
I met him all of once, at a Boskone, and saw him once or twice at other cons. I've read some stuff he put on Making Light, including Harry of Five Points, an iambic pentameter Henry V meets Guys and Dolls sort of thing that only someone like Mike would do.
I hardly knew him, and I am missing him so badly.