The old desktop system I was using for a server died last week, and I’m looking for a replacement, preferably a real, if low end server. I’d like to find something that is fairly low energy use.
Since I’m not really a hardware person, I thought I’d ask you guys for recommendations. 
Just for storage, or does it need to do things?
In general, “server” and “low energy use” are mutually exclusive, at least compared to something like a NUC’s energy use, although newer, low end servers are a lot better than they used to be in that regard.
I use it mostly as a file/print server, so it doesn’t need to do much. I imagine that just about any new-ish server will be more energy efficient than the old thing I was using.
Edit: It will have Active Directory on it, and will be the primary DNS server as well.
I kind of wish I could afford a cheap version of some of the Cisco gear I get to work on here: You could use it as a router, switch (NIM module) and run a VM for your server(s) (another NIM module is a UCS VM server),
That may not help, but is jsut the weird stuff I’d like to do.
Get a Pi!!!
A Pi won’t run MS Server 2012 with Active Directory! I also don’t want to depend on the roommate to configure it, since he’s the Linux person in the house.
Have you considered using a NAS instead?
You won’t be able to run MS Server as they have their own operating systems (usually Linux based), but I’ve found that isn’t an issue.
I have a Synology DS415+ and love it. It has an incredible number of uses:
You wouldn’t want to do all those things at once because it is a lightweight server - my one only has 2Gb of RAM. I have it doing files, media (that’s all media - video, music, photos), cloud, and sometimes Docker when I’m playing with that.
* Not that I’m saying I do that sort of thing of course, but Duckie TV is apparently a wonderful invention
I use Group Policy on my domain to control some desktop settings (printer mapping, login settings, for example) and I really like Active Directory to control user access. I have the occasional guest for game (tabletop) who logs into the domain to access character sheets or other game materials. I also feel that it’s a good thing to have the domain at home, so I can be an example to my students. I teach MS classes, after all. 
Hmm… The NUCs are a possibility. Lenovo is not very reliable in my experience.
+1 for NUC.
I have been running an i5 NUC as my media player for 18 months or so and it is excellent.
Also, +1 for a NAS (my NUC is paired with a QNAP TS-651. It does many very cool things as mentioned by @MikeP.
NUCs are good if you need compact and quiet, I’ve got an Intel one and it’s been rock solid.
My take home machine is a NUC. goes back and forth with me multiple times a month.
Another vote for a NUC, sized appropriate to your needs.
My guess for your requirements would be this one, which comes with a Broadwell i3 and can take a 2.5" drive and/or an M.2 drive. Cost right now is $291.
You’ll need RAM (8GB would be a good start) and a disk (a 120GB Samsung 850 EVO is a great fit for something like this).
Total cost before shipping/taxes would be $433.61.
If that’s too high, then you could take a whack at reducing your RAM (might save $10-20) or going with a lower-priced SSD (again, might save $10-20), but there aren’t that many cost reductions to be had here. You could also downgrade your NUC from an i3 to something less capable—see the NUC comparison chart for an outline of your options.
Good luck 
Sold! I upped the SDD to 250 GB, since it was only $5 more, but that should work.
Dang this thing is tiny. It comes with a bracket so you can mount it on the back of your monitor, and you’ll barely know that it’s there.
We just recently replaced our conference room computers with NUCs. We mounted them under the table so now it looks like there is just a magical wireless keyboard and mouse on the table. Pretty good specs too (Core i5, 128 GB SSD, 16 GB of RAM). Our IT director wants to do the mount behind the monitor thing when we do a hardware refresh in our call center. I’m all for it because those desks have limited space anyway. And also because most of the computers over there are POSes and the aforementioned NUCs would be a great upgrade.
That’s what she… oh… er…
Yeah, I live some of the tiny gear today, and I say that as someone with a couple 19" rack cases in my basement lab stuff. And as the guy who (mockingly) joked with the Mainframe admin at my previous job years ago because I no longer had the largest piece of equipment in the data center when her mainframe got moved in.
I use mine for work from home. Instead of a laptop I have a couple monitors and keyboards at home and the office. I throw the NUC in my bag and tote it around. All I have to do is keep track of my USB key.
Keep us posted on how the build & setup goes, @Nabiki!