In her latest post Alison Gianotto looks at a few different ways that you can validate any URLs that your users might give you to ensure they're not anything malicious. She looks at two of the major services, the Google SafeBrowsing API and SURBLs, as well as mentioning a few others.
If you write software for the web that allows users to submit or share URLs (comment systems, mail clients, forums, URL shorteners, etc), you may find yourself in a position where you need to filter out malicious links. Fortunately, there are several free options for you to better protect your systems and your users against bad guys, and they're pretty simple to implement. (My examples are in PHP, but could easily be adapted to whatever language you prefer.)
She starts with an example call to the Google's SafeBrowsing service, making a curl call and parsing the result. The other service, SURBL, makes use of DNS validation calls complete with code examples. She also talks about Phishtank and VirusTotal as other options. She finishes the post with a few suggestions for working with the rate limit restrictions on these services, including things like only checking on click-through and ensuring failure is handled well.