Very Compact Subwoofer

Anyone have an opinion on the performance of this sub:

Anyone know of other ultra-compact subs; must be 13" square or less,powered.

Velodyne's MiniVee, MiniVee 10 and MicroVee:
Totem Dreamcatcher
James EMB-1000
Are you looking for PC speakers - this is extremely small for a sub?

PSB-HD10 (HT sound)
Energy ESW-8 (HT Sound)
NHT-SOO (more musical - pro sound - less boomy harmonics that most people are so used to with subwoofers or if you are technical the group delay is kept low - few options on this sub as it is designed to go with M-00)
I could maybe use a "tube" subwoofer if it could be stood on end/vertically. Again, I'd be looking for it fitting into a less than 13" x 13" footprint.
sunfire subs are 13" cubed, tons of power at over 1000 watts they have a smaller version as well I believe
Martin Logan Dynamo is a great little sub.
I see there is also a smaller HSU Research sub; I've got some options now! Thanks, everyone, for the leads!
Douglas_schroeder, are you building it into a cabinet?

I'm very pleased with my Sunfire Super Jr, but it's a dual driver unit, so not an easy install if it's enclosed. Other than that I second the Velodyne Micro-vee series.
Don't forget MJ Acoustics and Rel for small foot prints.
Leedistad, No, I would not build these subs into a cabinet. I'll be using them standing alongside a pair of Eminent Technology LFT-VI speakers, which are wonderfully open and throw a big soundstage but need some low end augmentation. I would like to get the low end down to about 30 Hz realistically for such a small sub.

An Infraplanar sub would be ideal, but I don't want to spend tripple the entire system cost for a sub. Not going to put that kind of money into a second system.
Don't forget to check out SVS: SB-12 Plus

They also make cylinder subs which have a small footprint.
I believe the ScanSpeak Revelator 23W-4557T ten inch diameter aluminum driver will work in a 1 cubic foot sealed enclosure. Contact Rick Craig of Selah Audio, as he's built subs for me with these drivers that are phenomenal for music.
Velodyne SPL800 - small, tight, powerful. I use two and love them.
The Von Schweikert vs-1 is a tad larger than 13". It is a great sub. Easy to set up, solid, phases well with electrostatics as well as regular speakers. Better than Martin Logan dynamo, snell, m&k and comparable to REL. IMHO.
A follow up for all my helpful friends; I ended up purchasing two HSU STF-1 subs. They are the smallest full range sub HSU makes - 8" driver with 150W powered. They are (going from memory) 14L x 11"W x 19"H, which was the right dimensions for me.

The sound is extremely good, better than I figured for a powered amp 8" driver at $300. The volume and frequency response controls need to be dialed in appropriately or else it gets very sloppy, but the range is very wide and accomodates integration with the Eminent Technology LFT-VI speakers easily.

I'm very pleased with the performance, much better than I expected at the price point and size. As long as they are not overdriven or set up incorrectly they are a good solution to adding respectable bass below 30Hz in smaller environments.