Non-Linux FOSS: Chrome, for One
When I use OS X, I really like the Fluid app for making standalone Web applications. The problem is, Fluid isn't free unless you want the basic version. I don't mind paying for an application (and I did pay for Fluid), but it seems like something as simple as a single site browser shouldn't be something that costs money.
Thankfully, the folks at Epichrome feel the same way. They've created an open-source project at GitHub that allows you to create standalone OS X applications that use Chrome in order to provide a single site browser.
You can customize the way the app looks, give it a custom app icon and even register it as a browser on the system, so you can have it open when clicking on a specific Web site from inside your Web browser. I personally use it for my BirdCam, but it's a great way to turn any Web site into an "app" that you can launch from your dock.