Turn a Raspberry Pi into an oscilloscope. Probes attach to the GPIO pins.
A site that tests the capabilities of the VPN you're using for privacy leaks.
A service that logs into XMPP servers and tests their compliance with various XEPs. Generates a visual map of compliance on a categorical grid. You have to give it a user account on the server so it can log in and run tests. You do /not/ have to add your server to the public roster!
There is also a command line tool in Github.
A website that can extract many different sorts of information pertaining to IP addresses and networks, least of all querying several dozen blacklists to see if an address has been flagged as a spammer's.
Along with NIST SP800-22rev1 (statistical analysis of pseudo-random number generators), the Statistical Test Suite can be used to actually perform the tests outlined in this document. Runs under Windows, Linux, and Mac OSX. Written in C.
How to test an ejabberd.cfg file before restarting the server. Easier than it seems - why not make it a shell alias?
cat ejabberd.cfg | erl | grep syntax
If that command returns a syntax error line, you know there is a problem in the config file. If there is no output, ejabberd.cfg parses correctly.
Plug an XMPP server or client into this site and it'll audit certain aspects of its COMSEC posture, such as key sizes, whether or not crypto is enabled, and what crypto protocols it supports. If you're paranoid about instant messaging, you may wish to start by using this site.
A website that tests to see if arbitrary URLs are accessible from within China (behind Golden Shield/The Great Firewall).