I don't know how I let this gem slip past my radar for so long. It was only via a post by Dr. JTS himself (aka Martin Davis) that I saw a screenshot of JTS TestBuilder and decided to check it out.
I was actually just talking with someone about a tool that could provide simple visualization of WKT geometries; JTS Test Builder does that and much more.
You can input geometries (graphically or by well-known text) and compare two geometries based on spatial predicates:
Do overlay analyses with the two geometries. Note that you can see the result as WKT below.
And there are a host of other spatial operations to generate geometries using buffers...
... convex hulls ...
This app provides a very nice and user-friendly way to quickly and simply explore and test geometric operations. To try it out, download JTS and unzip the contents somewhere. If you're on windows, the .bat file is provided. If you're running anything else, you have to cook up a shell script that will set up the environment and run JTS TestBuilder:
for i in $JTS_HOME/lib/*.jar; do CP=$i:$CP; done
java -Xmx256m -cp $CP com.vividsolutions.jtstest.testbuilder.JTSTestBuilder $*