Have you seen/read Stars Without Number by
Sine Nomine Publishing? There's
a free version on RPGNow. The game itself is a Sci-Fi game geared towards sandbox style play and has lots of great world building resources (in fact, most of the publisher's works are loaded with such resources). Of note from Stars Without Number is the chapter on Factions.
The Factions chapter is included in the game as a GM tool for moving the world along around the PC's. It makes a faction into a character (hit points and all) that creates plot hooks and events that affect the setting to which the characters can respond. It has rules and stats for their resources, money and actions (and their costs) that they can take to bolster themselves, take down other organizations, expand their level of control, etc. The size and scope of these factions could range from local, national, planetary or intergalactic. Worth noting is it can also be a mini-game for PC's which have gained enough power to become their own faction.
Even if you weren't going to play Stars Without Number, this chapter (especially since it's free) might be a good tool not only for campaign building but also to create gameable experiences for characters who have become too powerful for adventuring, as well as offer tools for crafting value and cost for events at the table and in your campaign milieu.
