There are a lot of tools out there that will help any webdesigner or developer.
I use the following online tools on a regular basis.
Too me this one is a life saver. It helps you calculate font-sizes for baselining text.
Generate CSS3 code for border radius, box shadow, text shadow and more!
Specific tool that generates CSS3 gradient code.
You know you want to abide to web standards? The best online tool that works anywhere.
If I can't use Compass I prefer this tool to generate a CSS grid.
Not really a web app, but very useful. Copy and paste special characters.
Simulate JSON REST calls
Online web app to create favicons. By far the best I've stumbled upon.