Unless you jailbreak your iPhone to install Intelliscreen, your iPhone's wallpaper is static and pretty, at best. But two apps, Wallpaper Labeler and gCalWall Lite, let you add appointments and notes to the home screen.
With Wallpaper Labeler, you can enter any text you want to your wallpaper, so you can leave yourself (or whoever finds your phone) notes. The other app, gCalWall is more of what I'm looking for: It pulls down upcoming appointments from your default Google Calendar and splays them all over your home screen. You have to update it manually, which is kind of a drag, but the dev says that iPhone API won't allow for automatic updating =((((. Still, pretty handy. [iTunes, iTunes via Lifehacker]