Nikasil seems to be the coating of choice; I believe it's even what Cosworth uses on its F1 engines. It's a Mahle (the piston company) patented process, and involves embedding silcon-carbide particles in a nickle matrix. The silcon cabide is just short of diamond in hardness, and is both very wear- and seziure-resistant.
I've no experience with the particular people who advertise in the road racing papers, but Mahle use to offer the service itself, and I'd heard good things of TZ750 and TZ250 cylinders that had been recoated by them.