The concept of a single-site browser or site-specific browser (SSB, either way) is simple: give me a window with one website in it, preferably a desktop application replacement like Gmail, RTM, Basecamp or Zoho, and let that window behave like a regular application with its own Dock icon, notifications, etc. If you're spending a lot of your time on a particular site, this can simplify your life quite a bit; if you're mixing up GTD with ADD (as so many of us seem to be), an SSB can help limit your distraction horizon while you're trying to maintain focus and flow.
The inspiration for many SSB offerings was the Firefox offshoot Webrunner, and the descendant of that project has now earned a 1.0 beta designation and its own website: Prism, from Mozilla Labs, gives you a power tool for creating your own SSBs at will, either via a Firefox extension or by launching the Prism config app and typing in the target URL.
Aside from having a dockable icon for each website you convert, you can also set your SSBs to launch at login, or assign mailto: links to open your web email client (similarly achievable for Gmail with the Gmail Notifier tool). If you have to keep separate sets of credentials for work & personal accounts for web services, no need to log in and out repeatedly -- just set up a Prism SSB for one of the accounts, and the passwords & cookies will stay as they need to be. In my brief testing this morning, several sites worked just as expected; the only sticking point is that the Choosy extension gets confused about whether or not Firefox is running when an SSB is open.
Safari 4 developer seeds had offered a "Save as Web Application" feature for creating SSBs, which has been stripped from the File menu in the current public beta but still looks to be part of the final release; meanwhile, you can still make WebKit-centric SSBs with the excellent and free Fluid.
What site or webapp would you put in a single-site browser?
Thanks to everyone who sent this in.
[H/T to Lifehacker]