This page will continue to be updated over time.  If you have a link you would like to add, please send it to Thrakazog via private message.

Gateway University Library
http://gateway.pocketempires.com/
 
The GUL is basically a giant cross-linked encyclopedia of all things Traveller.  If you have a question about anything, look up the reference there.  Chances are you'll get your answer.
 
 
The Map Of Known Space
http://www.utzig.com/cgi-bin/iai/map_top.pl
 
This one is pretty self-explanatory.  Note how Traveller map detail is broken down into the units: sector, subsector, and system.  A sector is made of sixteen subsectors.  Subsectors are roughly 80 systems (8 x 10) in size.  Now, please do not start with me about proper mapping in three-dimensional space.  Somehow this works.  Just suspend your geeky disbelief.

This site also gives the viewer a good sense of the relative locations of the Imperium, the Vargr Extents, the Domain of Deneb, the Aslan Heirate, the Sword Worlds, and the Zhodani Consulate, which collectively make up the bulk of the galaxy political scene. 
 
 
Walt Smith's Traveller Page
http://users.hartwick.edu/smithw/traveller.htm
 
This site has the very best essays to date on piracy in Traveller.  His High Guard Battle Reports also give you a good feel for what starship combat is really like.  (Hint: it ain't pretty.)
 
 
Traveller Wiki
http://traveller.wikia.com/wiki/Traveller_Wiki

If you know what a Wiki is then 'nuff said.  If you don't, it's an encyclopedia very much like the GUL, but often this site also has pictures. 
 
 
The Traveller Webring
http://www.webring.com/hub?ring=traveller
 
 
The Traveller Deckplans Webring
 http://www.webring.com/webring?ring=deckring;index
 
 
The Traveller Gearheads Webring
http://www.ringsurf.com/ring/travgearhead/
 

Traveller 3D
http://www.traveller3d.com/

This site is great for depictions of the most common types of space craft you'll find.  There's a size chart to give you a handle on how big a pinnace is relative to a far trader, or an idea of the size of a 100-ton vessel, and so on.  A great resource!