Binary fills your screen with scrolling 1s and 0s, with a font and speed of your choice. Binary also supports DNA sequencing, which scrolls C/G/A/T instead of 1 and 0.
Download Binary (57kb Self-extracting setup)
A great font to use with Binary is SF Digital Readout. In any case, make sure to use a fixed-width font like Courier New, or it will look really bad.