"Adventure-seeker Killy in the Cyber Dungeon quest!" - Tagline from BLAME!
Tsutomu Nihei's BLAME! is one of the better examples of Seinin manga and is a prime example of cyberpunk manga as well. The main character is Killy, a man looking for people who carry Net Terminal Genes. It is only 66 chapters long and is a good read. There is also a CGI movie in the works...
"Maybe on Earth, Maybe in the Future."