The current release is : SimpleTest v1.0.1beta.
You can download this version from SourceForget.net and your local mirror. You may also want to look at the current changelog.
By the way, don't be scared away by the beta
tag :
quite a few users actually use SimpleTest straight from CVS.
If eclipse is your IDE / editor of choice, you might consider the Eclipse plugin.
For automatic "find and install", the URL is :
http://simpletest.org/eclipse/
SimpleTest has been packaged by the community to different flavours.
Note : careful, some versions are not always up-to-date.
The source code is hosted on Sourceforge as well : you can browse the CVS repository there.