With the iPhone having both WiFi and storage space, it only makes sense that there are apps that can take advantage of both of those features and make your iPhone into a mobile suitcase. DataCase is an app that does exactly that by connecting to your WiFi and offering you ftp, afp, and http connection paths to the remaining free space on your iPhone.
Once you connect to the address through whatever method you choose, you can easily view, upload, and download all of the files that you're storing. This makes it pretty handy when you want to bring something over to a friends house but don't want to lug your computer, burn a cd/dvd, or use a flash drive. You just need to make sure that you can get on to their network.
Your free space is always determined by exactly how much room you have left on your iPhone, so you may find that you need to clear up some additional room to carry any sizable files. Considering that this is only transmitting over WiFi, as long as you keep the app open you'll still be able to send and receive files, even while you're on a phone call.
There are several other apps out there like this. The difference between all of them seems to be in how they look. DataCase is very simple and straight forward. Considering this isn't really an app you'd be looking at the entire time you're using it, there really is no reason not to pick DataCase over any of the other apps.