Net-tops are essentially a new kind of desktop system that leverage the kind of hardware that was developed for the netbook market. They are in effect, a new kind of very-small-form-factor desktop. The Acer Aspire Revo is typical of this class of hardware.
Such net-tops tend to be based on the Intel Atom processor, which runs in the 1.2 GHz to 1.8 GHz range. It's most commonly a single core chip, but the higher end models are dual-core. It's a startlingly capable processor for use in an Asterisk appliance.
Net-tops can often be stuffed with up to 4 GB or memory and include a small internal hard drive. By small I mean physically a 1.8" or 2.5" drive form factor, with storage capacity typically in the 80-160 GB range. That's more than adequate for any Asterisk system.
Of course, using a hard drive in the system takes it out of the running of my personal definition of an "Asterisk Appliance." I accept that your definition might not be a rigid as my own, or, budget be damned, you may elect to replace the hard drive with a solid state disk.
In truth, these little net-tops are likely the most capable of systems being considered in my little overview. With list prices in the $300-400 range, they are also at the top of the price range that I was considering. But that's only if you look at list prices. Refurbished offers on the Acer AspireRevo AR1600 can often be found around $180.
The Acer Aspire Revo was actually brought to my attention by Ward Mundy of Nerd Vittles fame. On a recent VUC guest appearance, Ward indicated that the little Acer net-top was a very good platform for his PBX-In-A-Flash and Incredible PBX distributions. Both of these are "heavier" distributions than I might consider for an true appliance, but they do highlight the considerable capabilities of the hardware platform.
My own experience with net-tops involves a slightly more expensive device called the fit-PC2. This little box is essentially an industrial version of a net-top. With a 1.6 GHz Atom, 1 GB of memory and a 160 GB SATA hard drive, the fit-PC2 runs cool and draws only 10 watts, even under full load when playing an HD movie trailer.
I have had a fit-PC2 loaded with various software packages, including Astlinux v0.70. It can easily handle all the calling needs of a small office.
As you might surmise, the fit-PC2 is is the second generation of industrial net-top from CompuLab. For those who are more cost-conscious, their original fit-PC model has been renamed the fit-PC Slim and can be purchased for under $200. The fit-PC Slim shares the same 500 MHz AMD Geode LX CPU as the embedded PC systems considered previously, but comes in a nice compact little package.
Unlike most of the embedded PC boards the fit-PC Slim includes an IDE interface and a small hard disk, making it more like a small PC than a real embedded system.
This is by no means a complete survey of the possible platforms available for a DIY Asterisk Appliance. But it should serve to help you decide what might best suit your needs. Clearly some of the platforms described remain more appropriate for hobbyist and experimental applications than production use in a home, home office or small business. Even so, it's easy to see how for a modest amount of money, say $150-250, you can assemble an Asterisk system that will serve your needs reliably over the long term, and even accommodate growth over time.
There is some merit in trying to determine where you'd draw the line between something that's suitable as an inexpensive platform for experimentation vs. a production system. In my mind, this defines what would be considered an acceptable solution if recommended by a consultant or reseller for a small business or home office.
My sense is that the last three classes of hardware considered are completely acceptable solutions to real-world installations. I think that the thin clients, embedded PCs and net-tops are sufficiently polished and capable to be relied upon for everyday use. They don't give the impression of being experimental or hack-ish.
In contrast, the other platforms may actually be better suited to experimental applications. The very fact that such inexpensive, even inconspicuous hardware can be used to host a PBX suggests that there are many opportunities to communication enable previously silent aspects of our daily lives, learning something and having some fun along the way.