Light
Having some kind of artificial light at night is crucial. Finding gear, "answering nature's call", and checking your maps are three common things you'll use your light for at night. A tiny microlight will do unless you plan on doing some night hiking. For night hiking, a headlamp fits the bill.
Lightest Option
The Photon Microlight (seen here) is the lightest handheld light available. There are hat clips for the Photon Microlights that allow them to be used as headlamps. The lightest weight "true" headlamp is the Petzl e+LITE Headlamp. It weighs just 1 oz. To see the Petl e+LITE Headlamp, click here.
Cheapest Option
The Photon Microlights mentioned above are pretty inexpensive. For inexpensive headlamps, check Wal-Mart or Target.