There's nothing worse than opening the lid of your laptop, only to discover that it's been sleeping for so long that the battery is completely drained.
Smart Sleep is a preference pane that allows you to control whether your Mac goes into sleep mode or hibernation mode when you either shut the lid (or it's been idle for a while).
You can specify that it should always sleep, always hibernate, sleep then hibernate or "smart sleep". The latter lets you specify a percentage of battery capacity at which the laptop will start to hibernate.
One thing to note: you need to install the preference pane in the system folder ... /Library/PreferencePanes ... rather than in the one under your home directory. So, don't just double click to install it, copy the file to the folder manually.