I gamed twice in the last couple weeks, most recently in Chgowiz's OSRIC/1e game, which is modeled after the West Marches approach to group adventuring. So far we've played three sessions in that campaign, and the player mix has been different each time. In fact, Chgowiz and myself have been the only two static players; everyone else has rotated in and out.
Prior to that, I refereed the 7th session of Autumn Frontiers, my own sandbox fantasy campaign we're playing with the Savage Worlds ruleset. We've reached an interesting point in the game; several of the players are about to move into the "seasoned" experience bracket, which is my signal to start weaving a few of the disparate plot threads together.
True to form, Autumn Frontiers is set up like a traditional sandbox, with location-based encounters populating a largely unexplored wilderness setting. But each region of the map is rich in detail and mystery, and at this point the players have explored perhaps 30 percent of the whole wilderness.
I'm also handing over a few in-game tasks to the players. The shared table map, which I've been tidying up between games, is now theirs to use or ignore. It's got most of the main stuff penciled in already, but the rest is up to them. Same with dungoen mapping--next time we delve into some ruins, it's up to them to keep a running map of where they've been and how to get out. This particular task is quite a lot of fun, actually, as last Saturday's game with Chgowiz showed. My character has been mapping out the kobold-infested mines as we go along, and there's certainly a sense of accomplishment when the referee's description matches up with your own hand-drawn map.
In any case, that 30 percent (a relatively small area) has yielded up a lot of hooks, encounters and characters. There are a few common threads holding everything together, and over the next few sessions I'll start investigating how it all fits together. If the players show interest, we'll advance the plot together. If not, well, there's always another adventure waiting just over the next hill--quite literally in this case.