term-typing/operational semantics simulator?

I was reading Pierce, and was also toying with some type-system thoughts I'd had, and the idea working out a formal representation of such seemed like a fun deal, but rather than scribbling on paper, I was wondering if anyone knew of any sort of workbench/toolkit out there that existed for this sort of purpose. Something where you can enter terms and watch it go, see if you hit stuck states, etc. This'd be a magnificent boon to students, but I haven't heard of anything like this (I suppose this may simulable with coq or other sorts of high-power "expert-friendly" toolsets, but I haven't seen the "newbie-friendly" variant thereof...).

Has anyone heard of anything like this?

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PLT Redex

That sounds very much like PLT Redex.

thanks

I'll check it out. :)

OTT

I am not sure it can work as a simulator, but I think OTT is quite powerful to encode type systems and operational semantics.