Artha - Artha is a handy thesaurus based on WordNet with distinct features like global hotkey look-up...
Qikipedia - Instant Wikipedia summary cards on any web page
WordWeb - One-click lookup in any almost any Windows program; Hundreds of thousands of definitions and synonyms; The latest international English words; Works offline, or reference to Wikipedia and web references.
Six Degrees of Wikipedia - Find the shortest path between two subjects
GoldenDict - The program has the following features: Use of WebKit for an accurate articles' representation, complete with all formatting, colors, images and links.
Newsit - Find the discussion around the page you're looking at!