A couple of useful resources (I'll probably add more later) on a central feature of the genre: the franchise, with endless generally low-budget sequels, costs kept low by having few returning characters between movies able to demand higher fees (Scream was a notable exception), often treating the fanbase shoddily by selectively following the narratives of previous movies.
http://horror.about.com/od/horrorfranchises/Horror_Movie_Franchises.htm - Good list, with each entry hyperlinked to further info
http://uk.ign.com/articles/2006/12/18/top-25-movie-franchises-of-all-time-7 - slasher franchises feature in the list
http://www.joblo.com/forums/archive/index.php/t-32794.html - a forum's list of best 10 slashers ever, in which franchise entries feature heavily
http://www.toptenz.net/top-10-horror-franchises-they-should-have-killed-for-real.php - A different approach: the 10 franchises that should have been scrapped!
http://turtlecanyon.wordpress.com/2012/10/31/the-slasher-movie-franchise-rankings/ - a simple listing of the entries in 3 key franchises (Scream, Hwn, Fri 13th)
http://lovingthehalloweenfranchise.tumblr.com/ - a blog dedicated to Halloween (there are many more out there)
http://www.boxofficemojo.com/genres/chart/?id=slasher.htm - the top 20 all-time slasher movies by box office features ... 20 franchise entries, which says a lot about how the genre functions
http://forum.dvdtalk.com/movie-talk/606071-favorite-slasher-film-franchises-why.html - a poll and discussion about the best slasher franchise
http://www.he-man.org/forums/boards/showthread.php?179117-Slasher-Movie-Poll! - interesting list of best slasher killer etc
http://www.youtube.com/watch?v=VMJr9vSfZQ4&feature=related - most remakes have been abysmal; here's a video comparison of the orig + remake NoESt