I love programming in Python. I get paid to write Java though. Due to Java's verboseness, and lack of a REPL, this can be very frustrating for me.
In Python, the usual way I explore a new library or mock up a new idea is to immediately start coding in Python's interactive interpreter (or REPL). This is often times more efficient than reading (let alone finding) the documentation for the library. I can quickly see if something is going to work before I code inside my larger application.
Java doesn't have an interactive interpreter.. but Jython does! However, setting up Jython, especially interfacing with an already large Java application, can be difficult. One such difficult situation I deal with at work is in Weblogic. With Weblogic, I deploy my application directly to a running Weblogic server, and I never get to see a console in this process, so how am I ever going to run a Jython interactive interpreter?
This morning I got bored and wrote up a quick solution: JythonShellServer. JythonShellServer embeds into any Java application and starts a Telnet server that serves up Jython interactive shells. You can push any Java object that you want to manipulate into Jython's local environment. Run "telnet localhost 7000" and you can use Python code to explore your entire application's running environment.
JythonShellServer works, but I only just wrote it this morning, so consider it alpha quality at the moment. Check out the project page on github.