After awhile an iPhoto library and the Pictures folder can get quite cluttered with pictures from many sources. One way that can make this easier to handle is to use multiple libraries to keep things separated. I use three different ones myself. To create a new one or switch between different ones, just hold down the option key while opening iPhoto and a dialog box will prompt you for what you want to do.
However, switching between separate libraries using this method can quickly get to be a pain. I’ve written an AppleScript to help with this. Download the iPhoto Library Launcher, open it in the Script Editor, modify the libraryPath to refer to your library, and then save it. iPhoto won’t allow you to have multiple libraries open simultaneously, though the script should detect if another library is already open. If that’s the case, a dialog box saying “About to quit iPhoto!” will pop up and the user can choose to cancel or continue. If no choice is made within five minutes, the script continues launching the new library.