What are Mac alternatives to Storage Spaces in Windows 8/Windows Server 2012?
The short answer is not really. The more in-depth answer is that there's nothing directly comparable, but there are some alternatives worth looking at, depending on your needs.

RAID

The most basic option is using software RAID (either through built-in OS X tools or third party software) to create a mirrored (RAID 1) or striped (RAID 0) volume out of multiple disks. This is pretty limited in its flexibility, and to my knowledge there's no software that supports RAID 5/6 on OS X.

If you have a Mac Pro, you could get a hardware RAID card, which offers more options in terms of RAID levels, but is still somewhat limited for expansion, as well as costly.

External Units

The Drobo is somewhat popular for direct-attached storage. It provides   more flexibility in terms of mixing and matching capacities, but it has not always had the greatest reputation for performance.

NAS

Using a NAS, whether a pre-packaged solution or a custom built one running some variety of Windows or *nix gives you greater flexibility in most cases, and over wired ethernet can be faster than a Drobo. It can get more complex, and you run into the limitations of networked drives — in particular, Time Machine isn't really built to deal with network drives (it can be done, but gets a bit jacky), but it can also give you the greatest control over your configuration and future expandability.