Gamers are a special breed: inquisitive, dedicated, technologically savvy and often in the prime "young male" demographic that so many advertisers covet. With the rising cost of online games development, it was only a matter of time until advertising within and around games began to appear.

Various networks have been formed recently to facilitate the placement of advertising to reach gamers. Some networks deal exclusively with gaming sites while others focus on providing ads within the games themselves. If you seek to reach this group, a network in this category is much more likely to be cost-effective than an effort to make advertising deals with individual websites.

Examples include:
Future Games Network: game websites
Games Banner Network: computer game websites

Examples of in-game ad networks
Massive Incorporated
Eyeblaster
Atom Shockwave: sponsorships in Shockwave games
IGA

Example of mobile game network
Grey Stripe