This is a small collection of URLs and sites you can use to see if your IP or domain is considered "spammish" or not. This is not a complete list, but I'll update it as I find more useful tools.
This is an IP clearinghouse run by IronPort, and used by many commercial anti-spam systems (like, Barracuda). You can check your IP or domain status here.
There are plenty of RBL systems out there, and plenty of tools to check your status. Here are a few: