Homebrewed Sandbox campaigning can be a lot of fun and can keep any GM on their toes (the best place for any GM to be!). Spread out over the interwebs are tons of invaluable resources to help you with creating your Home-brewed campaigns. Here's a few that I've found very useful!
How to make a Fantasy Sandbox
Where to even begin you ask? Right here with Rob Conely's useful step by step guide with enough information to fill the heads of a room full of Ettins
Great hex mapping software. Pay version and free online app available.
Need that quick one-off or multiple dungeon level complex and no time to create one the old fashion way? Try your luck here!
Why take the time to roll up hirelings when they're just gonna get sliced and diced anyway?
Fantasy Name Generator
Coming up with names is a challenge for most any GM. This one has categories for books and tomes, ships, cities, castles, wilderness and many others for different settings. A must have for any Homebrewer! While your there check out the rest of the Donjon site!
Updated: Additional links from our posted comments: