The Google Safe Browsing API allows for URLs to be checked against Google's database of malware and phishing domains. However, on the Developer's Guide page, it says that the API is experimental and the data format is likely to change.
Although I have had good experiences with a number of Google's APIs in the past, they have all been fairly stable. I don't think I'm willing to use an experimental API in my project, so I'm looking for an alternative.