Apple has a couple of great devices in the AirPort and Time Capsule. Both can be used for shared storage, providing a centralized repository for files and backups through Time Machine. The two are very similar, with the main exception being that the Time Capsule has a built-in drive, whereas AirPorts require a USB drive to provide shared storage (a USB drive can also be used with Time Capsule either to back it up or to extend the amount of storage provided).

Both are much more limited compared to an OS X Server; however, there is one feature that, until Lion, they could do that OS X Server could not: Time Machine Server. In OS X Lion, the same functionality was brought into OS X Server in the form of Time Machine Server. You can back up to OS X Server without Time Machine Server, but provided you are on the same subnet, Time Machine Server actually shows the server in the list of available Time Machine targets without any additional work on the clients. Additionally, clients do not need to run a command-line hack each in order to back up to the Time Machine Server as they would if you were just backing up to a traditional AFP or SMB share.