One of the great production tricks with guitars is the use of the capo. Instead of a straight double, playing in a different register with the help of a capo provides a magic sound that seems to work every time.
But the normal capo works on all strings, and there are those moments when it would be cool to capo just one or two, and that's where Fretlocks very cool single string capos come in.
The Fretlock adheres to your guitar's fretboard with adhesive tape, and two small blades grab the string to fret it. It's way easier to watch and listen than read about it to understand, so there's a video below that shows what fretlocks can do.
Fretlocks are available from the company website and cost $22.75 for a packet of 6, or $37.90 for a tin of 12 (they come from the UK, so this is the current conversion from pounds). There are three different sizes to accommodate different string sizes.
The downside is that they're only usable for a few times before the adhesive wears off, but the company says that they're working on a more robust version for future release.
In the meantime, it's a small price to pay for having an extra finger or two when you need it.