The web today is not a benign place. There are many bots (automatic programs) that are constantly scouring the Web looking for a vulnerable computer. Once the bots find one, whether a home computer or a Web-server, they try to compromise the computer by surreptitiously installing its payload. In the case of a Web-server, it writes code so that the compromised Website is used for nefarious activities like Spam, Phishing and installing Malware on visitors computers. One of the popular way the hackers propagate their wares is by Drive-by-Downloads through malicious websites.
As an User: With reports of thousands of websites getting hacked now and then…literally many websites are hacked and infected everyday. Are you worried about stumbling onto a page that has been hacked and distributes Malware? or are you in need of information about a specific link that you received by email, which might take you to a Web page actively distributing Malware?
As a Webmaster: With gazillions of Websites out there, you may tend to think you are safe, that your Website would never be hacked…untill one day you find your Website blocked by a search engine. Even if your Website is not hacked, you may be unknowingly linking to Websites associated with Spam, Phishing and Malware, which will prompt the major search engines to flag your site as suspicious and dangerous.
The danger of not knowing what code lurks in your website while the search engine bots can still crawl and index your website might prove fatal to the credibility of your website. The malicious code may be a piece of hidden code iframes or spam links extremely difficult to find by an average Webmaster. Fortunately there are many free online tools that can help in checking any publicly available Website for the presence of malicious code.
I have made a list of such tools and services which should be useful for Webmasters and other web users alike.
Many of the tools also provide browser addon/extension that warns you before you interact with Web pages containing unwanted content and hidden threats. A list of them may be found at Freeware, Open source and Commercial Browser Security software.