There are a couple of good options, depending on what you are looking for.
The most obvious option is to use an Apple AirPort WiFi router to provide NAS:
- The AirPort extreme supports attaching an external USB drive
- TimeCapsule devices combine WiFi and an internal drive. TimeCapsule is intended to be used for backup with TimeMachine, but you can use all or part of your drive as a mountable volume (just like any NAS solution). TimeCapsules also have a USB port so you can add an external drive if desired.
Many other vendors sell Mac compatible NAS drives, but the reason I'm partial to Apple's drives is that there were driver compatibility issues when OS/X Lion first came out. Apple's drive were and are the most compatible (even if they are not the most cost-effective in $/GB).
If you are looking for a portable solution, you may be out of luck. The portable Wi-Fi drives, which are becoming popular nowadays, are intended as media "servers" for smart phones and tablets; they are read-only over Wi-Fi and require a USB connection to "load" music, movies, etcetera.