Free Apps - A SharePoint designers toolbox Fiddler
Allows you to inspect all HTTP Traffic, set breakpoints, and
"fiddle" with incoming or outgoing data.
IE dev Toolbar
Inspect your SharePoint CSS and a lot more
by Microsoft Firebug
FireFox Addon who lets you can edit, debug, and monitor CSS, HTML,
by Mozilla ColorSchemer
Online tool that let you put in an HEX an get a palette.
by Colorshemer Caml
Query Builder (windows version)
Produce CAML from your list and use it in XSLT.
by U2U Custom Workflow
Extend your workflow.