A lot of cool people have made some really cool sites with great information about Thingmaker, Creepy Crawlers, and Topper Superplastic toys. Here are a few that I know of. Let me know if I am missing any.
Creepyfun.com John Kolesar's extensive site with all kinds of great references, tips, Thingmaker art, and great photos of all the old Thingmaker sets. Also, some great techniques for finishing Creepy Crawlers/Thingmaker related toys.
MJ Thompson's Creepy Crawlers MJ Thompson's extensive site. excellent mold reference. He has scans of some of the instruction manuals that came with the original sets much more.
Retro-hobbies.com Ed's site that sells replica vintage toy stuff. Including Goop, new tongs, wings for bugs, hot wheels etc.
http://www.journalofantiques.com/Aug07/playing_around.html A fun article about Creepy Crawlers and "Make and Play" toys
World of Motorized Monster Maker Don's site is dedicated to the Topper Company's mechanical wind-up toys with monster bodies that you would cast with Superplastic. If you haven't seen them yet you should check it out.
http://www.klein-aber-fein.de/plastigoop Ralph's cool page full of German Plasti-Goop and Thingmaker stuff.
http://www.snowcrest.net/fox/thing.html Richard Hallock's Thingmaker pages. Lots of cool links and information. A goop section, Maker-Pak section and more.
http://www.geocities.com/garyweb65/thingmaker.html Gary's page with lots of nice pictures of the Thingmaker sets and Creepy Crawlers creations.
http://thehagues.net/cc_main.htm The Hagues's Creepy Crawlers Thingmaker pages. Some cool Thingmaker pages, Motorized Monster Maker pages, and Mattel injector info. They also have some scans of the booklets and paperwork.
http://en.wikipedia.org/wiki/Creepy_Crawlers Nice Historical info about Creepy Crawlers
www.plasti-goop.com New Creepy Crawlers molds and goop available
Didimo's Webpage Temporarily Down See Simone's tribute to vintage toys including the Italian Thingmaker!
www.toyadz.com This site has several of the old original Thingmaker ad's for sale.