While looking to grab screenshots for the blogs we'll feature on writefreely.org (https://discuss.write.as/t/want-your-blog-instance-featured/354) I remembered I already built a tool for @htmlhouse that'd help with this, called siteshot:
...and now I've gone down the rabbit hole of potentially launching a little screenshot-capturing service.
Not sure if I'll open that up to everyone yet. It works great for Write.as blogs, but results are mixed on other sites. If I can improve that, I'll launch it as a paid service.