Whether you're an aspiring writer or a seasoned veteran, you should definitely take a peek at this site. It's been around for a while, so most veterans out there probably have already checked it. But it is being updated all the time.
What is this site? This is a site that lets you know about the reputations of publishers and agents. Are they good? Should you sign with them? Here, some of these questions are answered. The list(s) include both recommended, neutral, and not recommended publishers/agents. More or less, it lists the publishing world, along with any relevant notes on each entry.
Why is this list indispensable? First off, it's run by a reliable source. Secondly, it's updated regularly. And, of course, the information there will help you to make good choices for your writing career. You definitely don't want to be jilted out of royalties or sign onto bad contracts. Read the lists! Make informed choices!