Bookmarklets/Favelets (browser independent)

tota11y - an accessibility visualization toolkit

Helps visualize the most common accessibility violations (and successes).

HTML_CodeSniffer

Detects violations of both Web Content Accessibility Guidelines (WCAG) 2.0 (all three conformance levels), and the web-related components of the U.S. “Section 508” legislation.

Visual ARIA Bookmarklet

Allows any sighted person to physically see the use of ARIA on public websites.

Jim Thatcher’s Favelets

Several handy a11y assessment favelets/bookmarklets.