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 definit
I'm not sure Lex/Yacc will be of any help. You'll just need a basic tokenizer and an interpreter which are faster to write by hand. If you're still into parsing route see Irony.
As a sidenote: have you considered PowerShell and its commandlets?
If you don't fear alpha software and want an alternative to Lex / Yacc for creating your own languages, you might look into Oslo. I would recommend you to sit through session recordings of sessions TL27 and TL31 from last years PDC. TL31 directly addresses the creation of Domain Specific Languages using Oslo.
As Anton said, PowerShell is probably the way to go. If you do want a lex/ yacc implementation then Malcolm Crowe has a good set.
Edit: Direct Link to the Compiler Tools
Still early CTP so can't be used in production apps but you may be interested in Oslo/MGrammar: http://msdn.microsoft.com/en-us/oslo/
Jison is getting a lot of traction recently. It is a Bison port to javascript. Because of it's extremely simple nature, I've ported the jison parsing/lexing template to php, and now to C#. It is still very new, but if you get a chance, take a look at it here: https://github.com/robertleeplummerjr/jison/tree/master/ports/csharp/Jison
Coco/R is a compiler generator with a .NET implementation. You could try that out, but I'm not sure if getting such a library to work would be faster than writing your own tokenizer.
http://www.ssw.uni-linz.ac.at/Research/Projects/Coco/