If you set the WABAC machine back a few year,s we can get to what is likely the smallest DIY Asterisk appliance ever undertaken, based upon the appropriately named little Gumstix family of boards. To my knowledge Kristian Kielhofner of Astlinux and Star2Star fame was the first to port Asterisk to these teeny little boards. He showed it at Astricon in 2005.
At the time featuring a 400 MHz Xscale processor, the wee Gumstix may not be the least powerful host ever selected for use with Asterisk. But measuring only 20 x 80 mm, it surely must be the smallest.
Gumstix Verdex 500
Despite its small size, the board was surprisingly powerful, especially if not pressed hard for floating point operations. Kristian's port of Asterisk was very experimental. I don't believe that it ever reached a release candidate.
Nonetheless, the concept of Asterisk on Gumstix ignited a lot of ideas about embedding Asterisk into other devices. For example, you could embed an Asterisk system into an existing SIP desk phone without requiring the complicity of the manufacturer. In so doing, you would have the ability to give the phone sophisticated internal dial plan capability with support for multiple accounts and protocols.
Even just embedding a SIP-to-IAX2 protocol translator would provide a definitive NAT traversal solution without resorting to using cheap Chinese IAX2 phones. The ability to run effectively run IAX2 on a premium SIP desk phone would certainly be handy for anyone with an Asterisk or Freeswitch based back-end and remote home office workers.
Over time, Gumstix have evolved to use ever faster processors, and offer a wide range of interface boards. They've become wildly popular with the amateur robotics crowd.
The one issue for this hardware is the lack of readily available cases.The various CPU and interface boards clip together in a fashion that, while not unattractive, isn't physically robust. To be used beyond experimental applications, they should be securely mounted into another device or a some kind of case.
2. Pogo Plug
The Sheeva Pogo Plug is an embedded Linux based hardware device initially intended for use as an embedded file server. Attach a couple of USB hard drives and it becomes a file server with media handling functionality.
The user community soon realized that it's also handy as a low-cost embedded Linux system for any application. With support from the manufacturer an open source community has grow up around the device, including an Asterisk distribution known as PlugPBX
You can purchase a developers kit for $99 that includes the basic Plug computer running a 1.2 GHz "Sheeva" CPU, which is based upon an ARM core. The system includes 512 MB of memory and 512 MB of on-board flash for persistent storage. External connectivity includes gigabit ethernet and some USB 2.0 ports.
A recent thread in the VoIP forum over at DSL Reports details one user's weekend project getting PlugPBX running on the Sheeva Developers Kit. While this certainly seems like an interesting project, there are a couple of things to be considered when choosing a Sheeva Plug as your host platform.
As the plug is based upon something other than x86 processor architecture, there are some things that it will not be able to do. Largely this comes down to the inability to recompile some parts of the code that might be involved in an Asterisk installation.
For example, if you are bandwidth constrained ,you may wish to use Digium's G.729a codec. You won't be able to run that code using the Sheeva Plug. Typical of a proprietary piece of software the G.729a codec is delivered as a binary installation with no source code. Thus you cannot compile it for the ARM processor in the Plug. The same thing is true for Skype-For-Asterisk.
However, these two modules are by no means universally deployed. Many sites will not require those capabilities, which means that the Plug may be considered. Personally, I find the "wall wart" form factor a bit of a turn-off, but it no doubt appeals to some people.
The Sheeva Plug and its larger brother the Guru Plug are interesting ideas for hosting embedded Asterisk, but seem to me to be exclusively in the domain of the hobbyist or experimentalist. Fine for home use, but not something that I'd expect to see deployed in a small business setting.
3. Seagate Dockstar
Seagate have licensed the Pogo Plug technology and offer it in the form of a product that they call the Seagate Dockstar. True to the original launch of the Pogo Plug, the Seagate Dockstar is intended to be a server accessory for one or more portable hard drives. It effectively turns them into affordable Network Attached Storage.
Since Seagate's target application for the Dockstar is very specific, it's not unreasonable to expect that it's hardware specs have been trimmed to keep costs in line. The device is supposed to list at $99 like the Plug, but it has been seen on BUY.COM for as little as $25.
No-one has yet confirmed that they have Asterisk successfully running on a Dockstar. However, even as I was working on this post, one of the regulars over at DSL Reports VoIP Forum posted a description of getting Open-WRT and Freeswitch running on the Dockstar. That could make the Dockstar the cheapest little experimental embedded PBX platform available at the moment.