The Traveller Map - fabulous interface See Charted Space in relation to the rest of the galaxy!
Supplement PO3. The Space Atlas - BeRKA's excellent maps
The Traveller Atlas - the subsector maps are shiny!
Stuff Onlines's The Known Universe - woefully incomplete but contains two rimward sectors I hadn't seen before, this site is the source of the blank sector grids I've put to work on my Gateway Quadrant maps
The Interactive Atlas of the Imperium - interprets the UWPs for you and produces jump data, website performance is a little spotty so if something doesn't work try again in a day or two
AB's Atlas of Charted Space - Also interprets UWP, though not as elegants as the Interactive Atlas. Uses ASCII to draw the subsector maps.
Library Data: Location of Known Sectors - sector-level resolution plotting out all known regaions, including the Zhodani core route and the Imperial rim expedition! That's a lot of friggin' sectors!
Underdeveloped Sectors - focusing on the region coreward of the Imperium (i.e. Vargr space and beyond)
Map of Traveller Charted Space - with extensive economic number-crunching
IISS Stellar Cartography - includes a map of the Jump-1 mains in the region encompassed by the 1st Imperium
Gypsy Comet's Zhodani Files - dotmaps of some Zho sectors
Federated Development Agency - fleshing out Hiver space and beyond
Don McKinney's Traveller Sector Information - No maps but lists all sources for canonical sector data that were published through 1999 or so.