I can definitely recommend GSAP from my own experience (we're happily using it in a bunch of projects). They've put a lot of attention to performance and the library is very feature rich (with timeline controls, splittext, etc.)
Actually, it's even free for most commercial uses too. And no, GreenSock has never sued anyone. The goal has been to keep it highly accessible, hold a trusting posture with users, and have a small funding mechanism that protects it from stagnating like most open source projects.
SCENE MANAGEMENT, PHYSICS, ETC.
GSAP is not meant to be a monolithic, opinionated framework in which you build your entire app/site like famo.us. You can build the way YOU want, and then leverage GSAP to animate pretty much anything JavaScript can touch; CSS properties of DOM elements, canvas library objects, generic objects, SVG, whatever. It's insanely flexible. It solves a bunch of browser bugs and inconsistencies too.
With GSAP, you don't have to keep switching animation tools based on your project. It doesn't try to own the rendering layer like famo.us does. It just makes animating a lot faster, more fun and less tedious.
That's not to say famo.us is "bad" at all - some people prefer their approach and it has its strengths for certain types of projects (I think they're going mostly after mobile app development).
Yes, GSAP has physics plugins (see the docs). Check out the fluid motion of Draggable which (unlike jQuery UI's Draggable) works inside transformed elements and honors momentum and complex snapping rules. http://greensock.com/draggable/
Velocity.js is cool, but it has different goals; it's only for DOM elements and isn't as full-featured as GSAP (runtime controls, sequencing, animating along a path, overwrite management, lag smoothing, synchronized architecture, etc.) Julian did a nice job with it though. More thoughts:
http://greensock.com/forums/topic/9449-new-velocityjs-animat...
LICENSING:
If a client hires you to do a custom project (web site, banner ad, whatever), no special license is needed no matter how much you get paid. The only time you need the license is if you're reselling your product to MULTIPLE customers. That's it. Everything else is free. With this model, a small group of power users who make $ off of GSAP provide a funding mechanism to make it accessible to everyone else and allow GreenSock to provide solid support and continued innovation. Check out the GreenSock forums to see how well-supported it is (and has been for years).
I'd encourage you to give the famo.us, Velocity.js, and GSAP a try and see which feels best to you. Have fun!
