If you're interested in the concept and execution of writing your own interpretive programming languages, or enjoy reminiscing and working with BASIC-style code then the BLuB project by Scott Lawrence will be of great interest. He's written a language similar to BASIC that runs on the Arduino and compatible boards with great success. It's all in one .ino file which is uploaded to the board as normal, then operated through the serial monitor. Scott has also provided code so you can run the interpreter without the Arduino on a Unix-style system.
Just like the personal computers of yore you enter the code line by line - and with BLuB you can save and recall programs from the EEPROM. It's a great piece of work, which you can find from his Github page. And for more, we're on twitter and Google+, so follow us for news and product updates as well.
If you're new to Arduino and want to join the fun, the first step is a solid board for your projects - our Freetronics Eleven - the Arduino-Uno compatible with low-profile USB socket, onboard prototyping space and easy to view LEDs: