Great Cow BASIC (abbreviated as GCB or GCBasic) is an open source compiler that allows you to program PIC (Microchip) and AVR (Atmel/Microchip, including the ATMega328P in the Arduino Uno) ...
This is a fork of the code and examples from "Make: AVR Programming" that I've addapted to work with the Arduino Uno R3 (I'm using the SMD variant, but it shouldn't matter which UNO R3 you're using).