Whether you intend to stick to online writing, or hope to branch out and (re)discover print markets, MediaBistro is a great resource. I've applied for jobs through their comprehensive job board, and most recently took the plunge and paid the $55 for a one-year subscription to the AvantGuild program. My main focus? Their "How to Pitch" series - you learn insider tips on breaking into the $1 per word markets.
Take a look: MediaBistro
And right now you can get a free magazine subscription, or just request a refund and chip $10 off the $55 price.