MOVING TO FRONT, ORIGINALLY POSTED NOV 26-MANY MORE OPEN ACCESS JOURNALS THAN I KNEW ABOUT! IF YOU CONTRIBUTE, MAKE SURE THE JOURNAL ISN'T ALREADY MENTIONED, AND INCLUDE A URL FOR THE JOURNAL HOMEPAGE (NOT NECESSARY TO FORMAT IT)
Philosophers' Imprint was the first mover here, and has been quite successful; more recently it has been joined by Ergo, another generalist journal. There are several specialty journals: I believe Journal of Ethics and Social Philosophy was first; there's also Journal of Modern Philosophy, and now Philosophy of Physics. I imagine I'm forgetting some. If you know of other online, open access philosophy journals, please post links in the comments.