Old topic is old....
So, ENWorld has a trio of systems mechanically related, in the What's O.L.D. is N.E.W. engine. Fantasy/medieval (O.L.D), sci-fi (N.E.W.) and modern (N.O.W. which just came out and should be compiled into a full gamebook any day now). It's about as sandbox-y as a system can get, each has a distinct flavor (sorry. It's author is from the UK, so "flavour"). Lots of crunchy bits to play with, as open an open gaming license as you can get.
Some things I like:
- magic and psionics aren't simply rehashed between OLD and NEW. Each system is mechanically different. No palette swapping, here. I haven't had a chance to see how NOW's "chi" works, just yet.
- the bones of the system are free: you can try before you buy, by visiting the site and scrolling/clicking through the various bits and bobs. There's even a bestiary.
Some things I'm not terribly fond of:
- open game is open. ANYTHING can be a skill.
- if you build it.... Well, you'll have to build it. A lot of it. Some of it, from the ground up. If you like a tinker-y, mess-with-the-system-under-the-hood approach, or like to get hip-deep in the world building, then it's a great system to sink your teeth into. But it's not exactly something you can just pull off the shelf and play cold.
Undecided:
- there are no classes. You build your character layer by layer, and what they do shapes them into what they become at the start of the game.
There are some freebie adventures available at ENWorld and the DriveThru W.O.I.N. storefront. More are available through the Patreon-supported magazine, EONS (though many of these lately, I think, have been made available through the DriveThru site)
Full disclosure: I wrote the adventure for EONS #56, "Incident at ICEREACH-1"