A common jazz reharmonisation, which lets you replace dominant seventh chords with the dominant seventh a tritone away.

This is easiest with the V in a ii-V-I progression. For example, you might substitute the C7 in Gm7 - C7 - F7 with F#7, a tritone away. This yields Gm7 - F#7 - F7, which is slightly more interesting harmonically and provides a good strong step-down in the bass.