I'll be working on blog roll in which both of these will be included, but for now I thought I'd just post them here.
First is "My Little Corner" at http://sandraseamans.blogspot.com/ . Sandra lists new markets and reopened markets before I see them anywhere else. She does her homework on them too and often puts them in perspective. And sometimes she posts writers-beware warnings when I market is up to something shady.
I also like that she doesn't stick to one genre. She focuses on crime, but you'll find plenty of horror, sci-fi, humor, and even romance markets posted. And she updates often.
The other one I recommend is The Horror Tree at http://horrortree.com/. Obviously you're going to find horror markets there but you'll also see sci-fi and dark fantasy posts. Anything that horror can be threaded into really. They also offer some encouraging series to motivate the young writer. The best part of their system is the calendar where you can see deadlines for upcoming periodicals and anthologies.
As this site grows I'll have a list of more resources and writer's blogs in the sidebar.