Joel Spolsky gives development advice that boils down to this: whatever you do, don't try anything different - you wouldn't want a competitive advantage or anything:

Last summer when we had a group of interns build Copilot , we had to decide what language to use for new code. I know that typically on new projects there's a long evaluation period where you decide what technology to use, along with lots of debates that include some crazy person actually wasting quite a lot of time evaluating Squeak and Lisp and  OCaml  and lots of other languages which are totally, truly brilliant programming languages worthy of great praise, but just don't have the gigantic ecosystem you need around them if you want to develop web software. These debates are enormously fun and a total and utter waste of time, because the bottom line is that there are three and a half platforms (C#, Java, PHP, and a half Python) that are all equally likely to make you successful, an infinity of platforms where you're pretty much guaranteed to fail spectacularly when it's too late to change anything (Lisp, ISAPI DLLs written in C, Perl), and a handful of platforms where The Jury Is Not In, So Why Take The Risk When Your Job Is On The Line? (Ruby on Rails).

Yep, be like everyone else, building on exactly the same frameworks - there's the ticket.

Alternatively, you could try the route Avi's taken with DabbleDB...

Ironically, it seems that in his own shop, he doesn't exactly follow his own advice:

Finally -- as to what we use -- Copilot is C# and ASP.Net, as I mentioned, although the Windows client is written in C++. Our older in-house code is VBScript and our newer in-house code is C#. FogBugz is written in Wasabi, a very advanced, functional-programming dialect of Basic with closures and lambdas and Rails-like active records that can be compiled down to VBScript, JavaScript, PHP4 or PHP5. Wasabi is a private, in-house language written by one of our best developers that is optimized specifically for developing FogBugz; the Wasabi compiler itself is written in C#.

Hmmm - a custom development language that is specifically designed to solve their problems. Hmmm.