The service at tiddlyspace.com has now ceased operation. It was created in 2010 by the Osmosoft team at BT, and generously operated for the community by them and their successors until December 9th 2016.
TiddlySpace has always been an open source project, and the code is available on GitHub so that anyone can host their own instance.
Please use the TiddlySpace mailing list if you have any questions.
The manuals have moved to https://spaces.xememex.com/ambit
The site has moved to http://acarvalho.tiddlyspot.com/
This page is hosted on GitHub at https://github.com/TiddlySpace/tiddlyspace.github.io. We welcome pull requests to improve it.