HNProfile

HNProfile lets you quickly glance at a user's profile without leaving the page you're on. You can quickly see a user's bio, karma, account age - plus quick links to their comments and submissions. Plus, if they have an email address or Github account in their bio (providing they've set up a Gravatar or Github avatar), you'll see their avatar too.

HNProfile is open-source and also available for Firefox and Safari
You can find links, plus the source code, at: https://github.com/timdavies/hnprofile