There are several. My favorite so far is called Plastic, it's the most backwards compatible of what I've seen. Some people who want a more advanced system use MegaZeux, but it's much more complicated and has an entirely different programming language.
Basically, Plastic does what previously could only be done with hacks and work-arounds: Player transport, Objects that change their own color, Custom controls, and random numbers.
It also adds things which would have been virtually impossible to do in ZZT, like a custom sidebar, changing anything into a programmed object, objects that can be stepped on, integer variables, and custom designs for any object. You can have fake walls that look like carpet, solid walls that look like paintings, and breakable walls that have big cracks running down the middle.
And all these extras use a Plastic-OOP that incorporates all the ZZT-OOP commands you already know! No need to start from scratch! Plastic even runs most ZZT games with no changes whatsoever!
Yeah, I'm thinking of switching. But I'll still use ZZT when I want a minimalist challenge.
