Vizit

According to their website, Visuwords™ is an "online graphical dictionary — Look up words to find their meanings and associations with other words and concepts. Produce diagrams reminiscent of a neural net. Learn how words associate. Enter words into the search box to look them up or double-click a node to expand the tree. Click and drag the background to pan around and use the mouse wheel to zoom. Hover over nodes to see the definition and click and drag individual nodes to move them around to help clarify connections."

I gave it a whirl by entering the word blue, and it immediately built a nice big web of associations:



The service is also free for use by anyone with an internet connection; no membership or other access fee involved. I also noted this on the website -- "Visuwords™ uses Princeton University’s WordNet, an opensource database built by University students and language researchers" -- so that's a fairly respectable source.