Alder - widely used. Like npearce said, Fender uses it a lot (and have for years, since it's a good bodywood). Many of the US Jacksons are also made out of alder wood - the RR for instance.
Many of Jackson's foreign made guitars are built from poplar - I've been told that alder and poplar wood are very similar tonally, but that poplar is cheaper, generally a wee bit lighter and doesn't look as good as alder.
Judging the sound is difficult, but when comparing to mahogany, I think the sound in an alder guitar feels more responsive and distinct (more aggressive attack) - with less bass but still without being too trebly. According to my experience, mahogany gives you warmer, bassy sound and a softer attack signature (which may make it sound fuzzier sometimes).
I'm partial to mahogany, but that's just my personal preference.
'bane