Where can I get a list of "bad" IP addresses that are likely to be used by attackers or spammers?

Is there an open source project or a list of "bad" IP addresses that are likely to be used by attackers or spammers?

What you looking for are open source Threat Intel feeds. There are a number of them you can subscribe to or likely query. Here are a handful I know of and should lead you in the right direction.

• Collection of long list of lists on GitHub.

• Emerging Threats

• AlienVault

• Zues Tracker

• Palevo Tracker

• Abuse.ch

• Malc0de List

• Binary Defence List

UPDATE: ideally I want it to be a service call so I can call it using HttpClient or with JSON from my web page and get a yes/no (I find it hard to believe that no one has yet created such a web service)

HTTP is too much overhead for such a simple task and does not scale that well. Such things are usually done with DNS and there are lots of such lists out there. Just search for DNSBL.