Snailgun is similar to a feature just announced for a future version of JRuby. Ruby scripts often have large startup times, especially if they require a lot of libraries. What Snailgun does is leverage a feature of the UNIX operating system (which Linux and BSD are types of) called fork. What fork does is make an exact copy of a process. So, if your script has to start very quickly, Snailgun can start a copy in the background and pause it. When you need a new copy, it will fork it and run that copy, starting the script instantly. Just imagine how quickly your tests will run with no startup times! Because it relies on fork, it's currently only available on UNIX-like systems (Linux, FreeBSD, etc).
Yet another great library from SeattleRB! ZenProfile aims to replace the stock Ruby profiler. It can be injected into any program when run, and will tell you what methods were called and how long they took. For example, it might reveal that the each method is taking 70% of the time your program takes to run. It'll also spy on your program and tell you which methods called the methods taking the most time. For more on profiling, see our article on the subject.