To display ACPI status: 'acpi'
To play around: 'acpitool'
For actually doing useful things: acpid and acpi-support
If your CPU has 'SpeedStep' or similar, you may want to activate the 'dynamic frequency scaling' in the kernel - you will also need support programs. I think there is also a user-space tool for doing the same work (of course only 'root' user can use it - but it's still a 'user-space' tool.)