The Mac Mini is a great size, but a.) it's a pain to set up, b.) it's expensive, c.) it requires that a monitor be plugged in to boot, and d.) it uses a laptop drive.
What I'd like, ideally, is a small (doesn't need to be Mac Mini-small) PC w/ an Intel Core Duo (or similar, but I like that chipset - being originally aimed at laptops, it's super-low power/heat) chip, a real HD, no funky hardware requirements, and a lower price than the Mac Mini.
The closest I've found is the Shuttle X100, which fits the bill perfectly except for the "cheaper than the Mac Mini" part - it's $1000 for a machine basically equivalent to the $800 Mac Mini.
(On a related note, does anyone know of any good places to find the relative power-draw and heat-output of the various cpus/chipsets? I know the Core Duo is ridiculously low when it comes to either of those metrics, but I'm not sure how it stacks up vs., say, one of the newer Celerons, or the Pentium D, or whatever. For me, power/heat vs. cost is the most important metric; performance is actually relatively low on my list since it's just for a home server. The Core Duo seems to hit the sweet spot where you get low power, low heat, moderate price, and high performance. It's slightly more pricey than, say, a P4, but it also draws something like 1/3 of the power, which makes up for the $50 price difference. I want a box that will run quiet and cool and not chew up a lot of power, ultimately, while providing at least reasonable performance.)
(If you're curious, it does add up. The Core Duo T2600 uses 125 watts at peak load, the Pentium XE 955 uses 286. I pay 19c per KWH per month. 30 days * 24 hours = 720 hours per month. That means that for each watt something uses, it uses .72 KWH/month, if I understand the term correctly. So that 125 watt T2600 eats up 90 KWH or $17/month in electricity. The 286 watt XE 955 costs $39/month in electricity. And of course, it gives off more heat, and needs more fans, both of which eat up even more electricity. The end result is that picking a good low-power CPU can theoretically save $20/month in power per box - and I have multiple boxes running in my "server farm", 24x7. Realistically they're mostly idle, and while idle the wattage difference is more like 75 watts, but even that adds up - $10/month per machine, which for me would be something like $360/year. We could talk about how shitty power prices are in NYC, but that's a whole other topic...)