There's a lot of neat small x86 stuff, but a lot of it depends on whether you're going low power or not.
If what you want is something like a Mac Mini but w/ higher end i7's your options are some Intel NUC or Gigabyte BRIX's which will be significantly more expensive than a baseline 2014 Mac Mini (but a bit cheaper when you match high-end RAM/SSD) - no TB2 or OSX (some ugly Hackintosh hacks withstanding) though. Gigabyte's BRIX Gaming is interesting since it has a GK104M based "GTX 760," which is amazing for the form-factor, but the trade-off is in relative cost and jet turbine fans. (The Alienware Alpha, Zotac EN760, and Asus GR8 are other "smallish" devices w/ dedicated GPUs).
You can get some really cheap/small Celeron-based NUC-sized machines (~$150 bare-bones) and throw in RAM/SSD for a very nice low-power Linux system. We use a lot of these at work at the moment, these are great.
Most of the fun stuff going on is with low power Bay Trail processors. Zotac has the C-series, which is between a NUC and Mac Mini in size, but also a Pico series (PI320) which is tiny, much smaller than the ECS Liva or the Minix Neo Z64 (which are also tiny). Also, there's the MEEGO-T01, which is the first Bay Trail HDMI stick I've seen.
My main desktop machine is an Asus Chromebox with an upgraded SSD and Linux installed. Had a hex-core 3.8GHz tower sitting right beside it, but I found I never booted it.
I spend most of my life in terminal windows, but it runs FireFox, the Gimp, and older games like HL2 just fine. Fan almost never goes on.
It's a M004U (AKA CN60). I should also have mentioned that I bumped the RAM to 6G (though 4 would have been fine). CPU is a Celeron 2955U, 1.8GHz dual core.
Fan's moderately loud. It seems to only come on when the GPU is stressed, rather than the CPU (playing 3D games, essentially). Doesn't come on when I'm playing movies or compiling code.
I run Arch with XFCE as my desktop, which I admit isn't very demanding - but it's zippy. Can't say what it'd be like under Gnome/KDE, but if I had to bet I'd say it'd be fine.
Installing Linux isn't for the faint of heart, but there's some good guides and it doesn't take long.
If it's desktop CPUs you are looking for, as opposed to power saving mobile models in desktop shells, I did get the HD-Plex S1 when it came out and I've been happy with it. No moving parts to break down or cause noise.
