When we tried Electro-Harmonix's Cathedral Reverb, we loved its Infinite Reverb mode for sustained synth pad-esque sounds. The Freeze Sound Retainer is a standalone pedal version of this effect.
It works by sampling your guitar, but unlike a looper it takes a snapshot of the signal running through the pedal at the moment you stomp, and sustains it until you reset the footswitch.
This creates a backdrop that lends itself well to post rock and electro music. The toggle switch lets you flip between Fast, Slow or Latch modes. In Fast mode, the pedal starts sustaining as soon as you stomp, and stops when you release your foot.
Slow mode gives a variable-speed fade in and out to the sustain, which, when you're switching between chords, is much smoother than the slightly unnatural on/off reaction in Fast.
The stomp switch can also be used in Latch mode, so you can take your foot off for long sustained periods. While you do get an effect level knob that controls the volume of the sustained tone, we'd like to have seen an option for controlling this with an expression pedal, too.
The Freeze effect is ideal for creating thick soundscapes and drones with your guitar - especially when you run it through other effects like wah, tremolo or delay. Yes, it's a specialist pedal, but it's great if you create a lot of non-traditional guitar sounds.