While the term "wireless external storage" is relatively new, Apple's Time Capsule and any router with NAS support has essentially been doing it for years now, so it's not like we're cutting new ground here. Still, we think it has a nice ring to it (courtesy of LaCie), and if you haven't invested in some kind of backup storage away from your local hard drive yet, now might be a good time to do so.
We just had a local hard drive fry on us the other day, taking with it about 3,000 photos from our Lightroom library. Luckily, we just backed up everything onto our local NAS a week earlier, so most of our stuff was fine. We can't imagine what life would be like if we lost all our photos from our library had we didn't. Probably something along the lines of self-loathing mixed with rage. Nerd rage.
If your router doesn't have NAS support or building a PC and using it as a server isn't your thing, you can also check out a third alternative; LaCie's latest router/NAS combo device - the Wireless Space.
The Wireless Space is literally a non-Apple Time Capsule, allowing for the additional of a few extra terabytes to your home network. Simply fill it up with your media and the Wireless Space will share your stuff across your network for streaming via UPnP or DLNA. Mac users can also use it with Time Machine.
Pricing starts at $229.99 for the 1TB and $349.99 for the 2TB.