I know that each piece of wood is different and two similar pieces of a same essence will have different weight.
But in general, for the same body shape and size which wood essence will tend to give a lighter body ? Mahogany or alder ?
Warmoth has pretty great description of different woods' characteristics.
Go to: http://www.warmoth.com:80/Guitar/Bodies/BodyBlank.aspx and click on "body woods" on the left. There's a ton of info about tone, weight and color.
Mahogany guitars are generally heavier, with alder being quite light. Because of this, alder gives a light bright sound, and Mahogany gives a deeper Les-Paulier sound.
Of course it depends what sort of guitar the wood is used in. Semi-hollowbodies and semi-acoustics are generally a bit lighter than solid-body guitars.