One of the more interesting aspects of the current edition of Dungeons & Dragons is the concept of prestige classes. Prestige classes are optional classes that characters can't just start their adventuring career in. They must instead build into the classes. For the most part, these prestige classes, at least in their original concept, are inherently tied to the campaign setting, allowing the DM to really customize the game. Someone who wants to be a secret agent can join the assassins, become a member of the shadowdancers, or join a special group that the DM himself decides. Below are a few new prestige classes of my own invention, which can be used for any campaign. Naturally, as optional rules, they can easily be used or tossed out, depending on the DM's whim and the flavor of the campaign.
The Arcanotech - An arcane spellcaster who combines magic and science to create unique results.
The Underground Agent - An elite scout and troubleshooter for the mysterious organization known as the Underground.