What's the best cheap (under $400) server for Linux?

Any recent,decent laptop with a broken screen! Pick one up cheap in eBay, or advertise locally. Ideal for 24/7 use, low power and quiet too! If you want to use is as your internet firewall/router, pick up a cheap PCMCIA ethernet card as well. If it needs more storage, add a USB external drive. Works for me.

Does build-it-yourself have any appeal to you, assembling a motherboard, case, RAM, and hard drive? I probably would not be alone in helping recommend some parts if you wanted to go that way.

@gmander A laptop as a server?

@Thammuz If there isn’t much load in it, it can work perfectly well as a server. I’m assuming that this is a home network like mine with only a maximum of 3–4 concurrent connections.

Does form-factor matter to you? You aren’t likely to get a new 2U rackmount for $400, but from the example in your link it doesn’t look like you need to fit in a rack or serve more than a few clients at once. If that’s the case, pretty much any old PC or laptop will do, and you don’t need to spend $400 to get an old PC that will provide more power than the little brick in your link. Of course, such a machine will also consume more power, at least when in use- but linux has made quite a bit of progress with power management lately. I figure any unit under 3 years old ought to suit in that regard.

Adding to @koanhead‘s answer, you can get a decent refurbished desktop for below $200 US. Geeks.com is a company that has given me good service. Just to pick one example, they have this right now:

HP Compaq dc7700 Core 2 Duo E6700 2.66GHz 1GB 80GB DVD±RW DL No Operating System Convertible Mini-Tower
$179.99

I am @koanhead here. Over the years, he and I have built many a Frankenbox from stuff that was either on it’s way to the dumpster or that had just been fished out of one. I have been through about half a dozen desktops over the years yet I have only paid a total of less than $800 for all of them. And even that old P4 Northwood I gave to a friend of mine still has more than enough power for what you need. Of course, it has the added bonus of being able to use the graphics card to fry an egg….

So I say that you don’t even need a new computer, nor do you need to spend much if any money on it.

@jaytkay I could (and have) done better in some brick-and-mortar stores here in Seattle. I don’t know if you or the OP have similar places where you are, but it’s worth looking into.

+1 @gmander idea is excellent. For the services you listed a laptop will have enough horsepower and is inherently built for low power ops. You may have to monkey with the power management features to make sure it’s awake enough to avoid any latency issues.