Do a reverse lookup on the site's IP and add matchers for the hostnames that show up.
Mention if they're round robining their DNS.
Add nmap OS fingerprinting.
Do a traceroute and log the IP of the closest router (final hop) to the site and add matching for that.
Add a wiki interface and build a crunchbase like app.
Add archiving of data and monitoring over time (as netcraft did in their original app).
I wrote a Ruby program to do something similar to what you're doing: https://github.com/jpf/domain-profiler - If you ever start profiling sites using information from places other than what the server returns, perhaps what I've done can help inspire you?
After clicking the 'Add It!' link:
The wording on the previous page had me expecting a page for manually adding technologies to my site's stack a la Bagcheck.com.
More importantly, I like that users can easily add their own technologies and matchers.
Your analyzer mistakenly detected my website using Microsoft IIS and ASP.NET. Which is weird since it also detected Google App Engine (which is correct). ASP doesn't run on GAE.
Just a thought, do you think website owners should mention technologies they use in HTTP header? For example your analyzer can't detect that I'm using Java and Spring framework.
I could reach them with no problems. These sites are just from some comments on this thread:
Now, the following is a hack, but CSRF stuff gives a good indication:
If there's a form, there's a good chance you'll find a construct like:
<form action="." method="post"><input type='hidden' name='csrf_token' value="thetokenvalue" />
You will also most probably find the jQuery function that sets X-CSRFToken on XHRs (see the doc at #ajax). For prototype it'll look like this 
All you can tell from it querying itself is that the work happens somewhere behind rails.
Some bugs in the information returned though.
For example: http://underthesite.com/technologies/WordPress-Batcache-Plug...
"WordPress Batcache Plugin is closed source. "
Batcache is Open Source :)
In terms of features, I'd love to see more emphasis on the aggregate/comparison data. For example, most popular server side framework, most popular JS libraries, most popular hosting platforms and so on.
I like it a lot :)
Thanks for the feedback!
I've been using a Firefox add-on for this, http://wappalyzer.com doesn't give as much info, but works fairly good.
Though cannot reach this site: http://www.olin.wustl.edu/pages/default.aspx ...why
Under the Site misses MooTools, which is being loaded asynchronously on my site via Google Libraries API
Impressively, Built With even managed to detect that I was making an AJAX call to StackOverflow's API.
Even Adobe.com, a ColdFusion site, doesn't list it.
or libraries like flourish