American Association of Variable Star Observers: http://www.aavso.org/ Easily the biggest, most important and most internationalised variable star orginisation in the world. You don't have to be a member to make use of its extensive facilities, information, and databases - and you really have to if you are serious about variables. More about AAVSO.
AAVSO have several alert lists and email groups.
British Astronomical Association, Variable Star Section: http://www.britastro.org/vss/ World's oldest (and some would say highest-quality) amateur-based variable star organisation. Their website is worth a thorough investigation.
Center for Backyard Astrophysics http://cbastro.org/ Runs monthly campaigns on southern (also northern) cataclysmic variables. See website for current targets, observing and data submission requirements. It's kept as simple as can be so that anyone with a CCD can contribute - but don't expect any feedback. Join their CBA-Chat and CBA-news e-groups - very valuable.