If you have a hankering to convert some really, really old software written in BASIC to run on your Arduino you may want to consider Mike's port of TinyBASIC from the 68K assembler version. Although it isn't completely featured, you can get it working and control via the serial monitor. The project has also been picked up by another enthusiast - and TinyBASICPlus was born which adds SD card and LCD support, amongst other things.
You can take this project one step further by adding a PS/2 keyboard interface to your board for easier data input, and the instructions to do this and other fun projects are contained in the fascinating book "Practical Arduino" by Jonathan Oxer and Hugh Blemings:
Create your own Arduino-based designs, gain an in-depth knowledge of the architecture of Arduino, and learn the easy-to-use Arduino language all in the context of practical projects that you can build yourself at home. Get hands-on experience using a variety of projects and recipes for everything from home automation to test equipment. For more information and to order, click here.