HTML5 powered Hardware Communication Bus using a microphone and headphone port.
Maximum speed (as of right now) is ~45ms. This gives 1 byte every 45ms = 177b/s, or ~22 Chars/s. This could easily be boosted to 2-3x by modulating multiple frequencies for different values.
E.g: 2.5Khz and 4.5Khz = 'x' vs the current 2.5Khz = 'x';