Actually, maybe not full-blown Lex/Yacc. I'm implementing a command-interpreter front-end to administer a webapp. I'm looking for something that'll take a grammar definition and turn it into a parser that directly invokes methods on my object. Similar to how ASP.NET MVC can figure out which controller method to invoke, and how to pony up the arguments.
So, if the user types "create foo" at my command-prompt, it should transparently call a method:
private void Create(string id) { /* ... */ }
Oh, and if it could generate help text from (e.g.) attributes on those controller methods, that'd be awesome, too.
Answer
I've done a couple of small projects with GPLEX/GPPG, which are pretty straightforward reimplementations of LEX/YACC in C#. I've not used any of the other tools above, so I can't really compare them, but these worked fine.
GPPG can be found here and GPLEX here.
That being said, I agree, a full LEX/YACC solution probably is overkill for your problem. I would suggest generating a set of bindings using IronPython: it interfaces easily with .NET code, non-programmers seem to find the basic syntax fairly usable, and it gives you a lot of flexibility/power if you choose to use it.