Internet Explorer Developer Toolbar
This little gem is an invaluable tool for any web developer. "The Microsoft Internet Explorer Developer Toolbar provides a variety of tools for quickly creating, understanding, and troubleshooting Web pages." Here's a list of the features from the download page:
-- Explore and modify the document object model (DOM) of a Web page.
-- Locate and select specific elements on a Web page through a variety of techniques.
-- Selectively disable Internet Explorer settings.
-- View HTML object class names, ID's, and details such as link paths, tab index values, and access keys.
-- Outline tables, table cells, images, or selected tags.
-- Validate HTML, CSS, WAI, and RSS Web feed links.
-- Display image dimensions, file sizes, path information, and alternate (ALT) text.
-- Immediately resize the browser window to a new resolution.
-- Selectively clear the browser cache and saved cookies. Choose from all objects or those associated with a given domain.
-- Choose direct links to W3C specification references, the Internet Explorer team weblog (blog), and other resources.
-- Display a fully featured design ruler to help accurately align and measure objects on your pages.
I've been using it since beta 1 and it's definitely a productivity booster. There is a nice little feature not listed on the download page for those of you whom 508 compliance is a concern. Have a look at the 508 option under the Validate menu.