While building applications, games, css/xhtml Web sites, etc. it often becomes evident that a new tool is required. Just like some early inventor realized that pounding a nail with a rock was less efficient than it needed to be, the keen developer will come up with shortcuts.
Escape Characters - Web
>> Download or try it
Web Developer extension for Mozilla Firefox - Firefox
Wow! This is a must have for anyone who develops Web content. If your primary browser isn't Firefox then this might be the reason to switch. The Web Developer extension for Mozilla Firefox and Mozilla adds a menu and a toolbar to the browser with various web developer tools. It was developed by Chris Pederick and has useful features like: view with/without styles, switch window size, show/hide images, outline all block level css elements (!!!), validation checks and tons more.
>> Check it out at chrispederick.com
Favelets - Web
>> Check them out at favelets.com
Web Windows - Web
Speaking of sizing your browser window to 800x600 to see how your site will look, years ago I created these graphics that I set as my desktop wallpapers. When I needed to check out how a site I was developing looked at a particular screen size I would just size the window to fit into the outline on my desktop. Not particularly useful now, but I still get a few downloads of these every week so I thought I'd keep them around for now. Remember to set your desktop image NOT to stretch.
>> Download for the Mac
>> Download for the Windows