Search giant Google has launched a system that allows web users to add annotations to sites that other users of the system can read.

SideWiki, which is available from today as a feature of Google’s Toolbar add-on for the Internet Explorer and Firefox browsers, uses an algorithm to decide which annotations are most “useful”, and makes them appear first. It also recognises identical content across different websites, and associates relevant annotations with all instances of that content.

The system could have significant consequences for corporate websites, by giving existing customers to ability to share their experiences – good and bad – with potential customers. Engaging with the community that uses SearchWiki could therefore become an important PR function.

SideWiki may also help to combat the increasing number of web-based security threats, by allowing users warn fellow surfers of infected websites or downloads.

Similar services have been attempted in the past, but Google’s scale and brand-recognition makes SideWiki the most viable one yet.

Update – Already, the potential pitfalls for businesses are becoming clear. Tim Larkins, a Unix administrator working for Carphone Warehouse, has used SideWiki to share his personal view on www.microsoft.com, describing it as "one of the most useless websites I have ever visited."

What has also become clear is that Google will need to work hard to remove spam from the system if it is to be widely adopted. There are already a number of SideWiki messages supposedly advertising various services.