Astronomy Facts - Astronomers, Nearest Stars, Constellations, Messier Objects, Obsolete Constellations, and more (Shane Horvatin, Abrams Planetarium, Michigan State University)
Astronomy Thesaurus - Click on the British flag to get started in English. One you select a term, you'll see it in French, German, Italian, and Spanish at the top of the page. Entries show Use, Use For, Broader Term, Narrower Term, Related Term (although most entries don't have all categories). There is also a hierarchical listing.