Mac OS X has a couple of cool tricks for helping you focus on just one application at a time. One of them is the "hide all other programs" trick.
To hide windows from all other applications, and just show windows from your current application, type [Option][Command][H]. Instantly all the other windows are hidden.
That's great if you want to hide all other applications but the one you're currently working on, but let's say you want to switch from what you're working on (such as typing in TextMate) and you want to show all your browser (Firefox, Safari, Camino, whatever) windows. To do that just do a [Option][Command][click] of the browser icon in the dock, and -- bam -- all windows from the browser will be opened, and all other windows from other applications will be hidden.
Fortunately both of those keystroke combinations begin with [Option][Command], so hopefully they're easy to remember.
Also, if you haven't done this before, your other application windows are now "hidden", so you won't see their icons on the right side of the Dock. To get back to your other windows just click their application icon in the left side of the Dock, and they'll come right back. (And while I'm in the neighborhood, you can tell which applications are open by the black triangles underneath the application icons, again on the left side of the Dock.)