[dangrossman]

Take domains from here: http://yclist.com/

Plug them in here: https://builtwith.com/

You'll learn a lot from that, even though it can't peer too deeply into the backend of an arbitrary website. Lots of startups have tech blogs these days where they may have written about your stack; barring that, asking is probably the best way to get more details.

[citizenkeys]

yclist is at least a year out-dated. Should use my list instead: http://ycuniverse.com/ycombinator-companies

If you get your project off the ground, send an email to [e-mail] and I might write about it.