One of the most interesting, clever and powerful developments in the spreadsheet arena recently has been Resolver One by Resolver Systems, which blends a familiar spreadsheet-like interface with the powerful Python programming language.
Being build upon the robust web programming foundation of Python resp. IronPython the product is consequently offering a Resolver One Web Server too, which allows you to turn your spreadsheets into web applications with just a few clicks.
Of course the product features Drop-down in cells too: I've been able convert their related Drop-down cells sample into a web application within ~5 Minutes. Be sure to read the short Resolver One Web Server Documentation and don't forget to make the drop-down cell web editable before publishing the sample as a web application, as demonstrated in the Web Application in One Minute screencast.
The only downside of this is that you might need to host the service yourself: while I could easily imagine there will be hosting offerings around this amazing product in the future I'm not aware of any as of today.
One minor caveat: despite them demonstrating their web applications within Firefox the drop-down sample worked flawlessly in IE only for me right now;
I haven't investigated this any further though and would suspect it to be easily fixable by yourself and/or via the official support.
I've been contacted by official support regarding this: meanwhile they confirmed the issue as being session/cookie timeout related (i.e. it works initially in a fresh Firefox window, but after some time ceases to do so) and are working on a fix.
In case this product has sparked interest you might want to have a look at Resolver Hacks too, a site which explores various ways to get the best out of Resolver One.