For months we have been getting a message on the screen saying our storage was maxed out. And sure enough - I 'd go to "usage" in "Settings" and learn that we had 0 bytes available out of 6+ gigabytes! The problem was that I couldn't figure out what was taking up all that storage space. No photos, no songs, etc. I wiped out a year's worth of old emails - didn't make a dent. The list of apps added up to scarcely a gigabyte. I couldn't use the camera - no space. It was a mystery. But then it got worse. Mail started crashing. Safari started crashing. Blogger would crash. Even Notes! The phone was becoming almost totally neurotic.
So today I took extreme measures. I made a paper copy of all my phone contacts and voicemail numbers, just to make sure I wouldn't lose all that data, and I went into Settings and clicked on "Reset iPhone." That wiped out everything. Then I hooked the phone up to the computer and restored it from my last backup which was last December. That did the trick. I now have almost 5 GB of storage space. I lost recent text messages, etc., but everything works! Yay!