A NUC or micro pc is ideal for cramped spaces, htpc, digital signage, etc. They are tiny, but they can also be quite powerful going up to i7, although not a replacement for a desktop EDIT: due to lack of discreet graphics card and expansion options.
The two main players are Intel and Gigabyte, and both are on sale at CentreCom.
Gigabyte Brix: http://www.centrecom.com.au/gigabyte-nuc-brix?preview=true
Intel NUC: http://www.centrecom.com.au/intel-nuc?preview=true
If you miss out you can always get them for a few dollars more at PC Case Gear's normal prices:
http://www.pccasegear.com/index.php?main_page=index&cPath=15…
EDIT: Forgot to mention they do not come with OS, you need an extra ~$80-100 for win 8.1 retail, unless you can get an upgrade somehow from win 7, I think there are still ways to do it
They can absolutely be a replacement for desktop. We've deployed 50 NUCS on an office level block in cbd melb, and have been humming along nicely for almost a year. Asus also have their Vivo PCs as a contender.