Nkrs
Io is a really interesting language. It is very easy to master and if you want a weekend project, it is also very easy to make a simple Io-like language interpreter (I'm actually borrowing some of its ideas for my language project). Sadly, it isn't very popular - the main implementation is buggy and not very fast (comparing to something like Lua or Ruby or Python - and for a very good reason: Io is a simple AST walker while these have optimizations). Still, it's a very interesting project. There exists a language called Ioke that is very similar to Io, but runs on the JVM -
ioke.org