This was an intriguing experiment undertaken over at Minimal Mac: Patrick decided to play a little Homescreen Survivor with his iPhone app icons -- he cleared everything off his homescreen, and then only let those apps back on which he used more than once throughout the course of a week. The result? You can see here which ones made it back on. Contacts, Calendar, Camera all showed up, not surprisingly. Photos did as well -- I don't use that one much, but I can see why. Everything else seems to be his own personal use: Tumblr's app, two different Twitter apps (Birdhouse and Tweetie), and then Simplenote and so on.
But the real point here isn't what apps he used, it's just how different his homescreen looked when he only put what he used on there. I'll admit it: my iPhone is a mess -- I've got icons all over the place, and they don't seem to stay organized even when I try to organize them. But organizing according to actual usage is a great idea, and undertaking the same experiment on your iPhone might actually clear some things up for you.
It's also worth pointing out all of that empty space on his homescreen: he has four more slots there that aren't used at all. Apple, are you listening? I'd guess Patrick's usage is more common than Apple may think -- there's plenty of room on the homescreen for other functions and information.