Jeff VanderMeer is compiling a list of science fiction/fantasy manifestos at his discussion board. It's becoming a nice collection of weird proclamations, a one-stop resource for rants and raves.
The one that was new to me and that I was grateful to find out about (from L. Timmel Duchamp) was Jeanne Gomoll's "An Open Letter to Joanna Russ". Be sure to read it if you haven't -- it's one of those pieces that, regardless of whether you fully agree with it, deserves to be read and thought about.
(The SF world certainly isn't the only one that seems to spawn a new manifesto every week -- for some background on the artistic tendency to get all manifesto on the world, check out Manifesto: A Century of Isms. If you're a political revolutionary looking for a cheap collection of subversive material [and who isn't, really?], I know of nothing better than the Dover Thrift Edition of The Communist Manifesto and Other Revolutionary Writings -- it gives you Marx, Marat, Voltaire, Paine, Proudhon, Kropotkin, Bakunin, Emma Goldman, Gandhi, Mao, Guevara, Havel, and others all in one book for $3.50. If that won't get you storming some barricades, then you're probably dead.)
Manifestos and rants are fun! Everybody should write them -- and often! Down with the system that discourages manifestoing!!!