Tiny
In my second year of undergraduation, I wrote a compiler and interpreter for a simple expression based language called Tiny. Both compiler and interpreter are written in Java, and use a recursive descent parsing method.
Both parts were based on Jack Crenshaw's "Let's Build a Compiler" series, and were rewritten in Java (originally in Pascal).
It is a very simple program that can be completed in a few hours, but back in my second year of undergraduation, it sure seemed cool!