The cheetah was a good logo choice for this project. I stumbled across this compiler in the Ubuntu repos when I was writing a set of language benchmarks, and was completely blown away by how this small compiler produced some of the fastest results, even out-pacing the venerable C++ (particularly in string handling) in executables it generated.
GNU Smalltalk is a really easy interpreter to use (I tried Squeak, but found it overly complex for simply learning Smalltalk).
I used it to start learning Smalltalk, and found the language itself to be quite fun (again, because of Squeak's complexity, I couldn't get into the language at all using it).
Performance is surprisingly fast - much of the functionality rivals more recent languages, most of the time out-pacing Ruby and even PHP, and even being competitive (speed-wise) against Python.
Every bit of Java code I tried through BeanShell executed 100%.
It does suffer from a bit of a speed hit, but this is to be expected in scripting environments, and it didn't make it any less usable.
The interactive interface is a wonderful tool - it's an amazing tool to use when trying to do quick little tests. It's already become a pretty standard part of my development process.
This is a good Python implementation, but it is noticeably slower than C Python. It does have an issue in constrained permission environments, as it does try to instantiate a custom ClassLoader (presumably to compile Python source to JVM bytecode to boost performance). This is probably not an issue for the vast majority of usages, though, so the issue will likely be moot for most users.
This language is nothing short of awesome. I initially came across it when looking for an alternative to Ruby, and discovered a language which had all the strengths of Ruby, none of the short-comings, and added a whole lot of new features.
I expect Groovy and Grails to become as big (if not bigger) than Ruby on Rails.
Because of it's close link with Java, it opens massive possibilities for extending the language and providing high-speed libraries - something which is very difficult to do in Ruby (with the possible exception of JRuby).
One of the greatest possibilities of Groovy is that IDE's such as NetBeans allows you to mix Groovy and Java source in a single project, which means you can mix-and-match agile development through Groovy and high-performance components through Java.
Definitely a A++ project!
JRuby represents a Ruby implementation which is better than the original - it allows you to use Java libraries in a Ruby application, and typically out-paces C Ruby in every respect. It also simplifies Rails deployment (no need for running multiple instances of a server, as it can run inside a J2EE container).
The only problem I had was with database access - the adapters aren't quite up to scratch with the C Ruby libraries, but I expect this to stabilise.
Java integration is really good - you can configure the runtime such that limited permission environments are easy to cater for - the BrowserScripting project illustrates this by allowing Ruby scripting from within an applet (probably the most permission constrained environment you can have for Java).
I would give this a 5/5 rating, except for my personal dislike for the Ruby language itself.
Ruby as a language has it's place - and people need to remember that. I honestly can't count the number of times I've had people cram the idea down my throat that Ruby is the alpha and omega of programming. This simply is not the case. It's great at what it does - SCRIPTING! (be it small utilities or web pages). I certainly wouldn't want to use it to write a RDBMS engine.
It's not quite as "English" as people would claim (e.g. how "