A thread in the Google Web Search Help forum indicates that Google has moved the similar sites link found previously near the cache link in the search results.

Likely a result of the search engine’s recent relatively low profile site redesign (where it also dropped real time search – likely a result of Google+, the popular wonderwheel and image sizes) Google actually moved the similar pages feature to instant previews.

By selecting the magnifying glass icon next to the site title, the instant previews window will appear and a “Similar” link will appear at the bottom of the popup window. Web professionals can also still use the “related” search query (related:www.websitemagazine.com).