Google related: is an advanced search operator for finding websites similar to the specified webpage. For instance, the query related:labnol.blogspot.com on Google will list web pages that are similar to the Digital Inspiration home page.
I ran a few related: queries on Google and though the results were appropriate, the Google "Similar Pages" algorithms still need a little bit of tweaking.
There is another good website recommendation service called EasyUtil but they only provide an web service API so it's out of bounds for most of us.
Thomas Hawk informed me of a new search engine called Similicio.us that allows you to type in a URL and receive suggestions of similar domains based on del.icio.us tags/bookmarks. Great idea.
People generally bookmark sites with good niche content on del.icio.us. Therefore, results of similicio.us are more relevant and useful.
Thomas adds that since Similicio.us uses Del.icio.us tags in order to find similar sites, it excludes a lot of smaller sites that may not have shown up in the mainstream blogosphere just yet.
Similicio.us is also designed by Ying Xie of EasyUtil.com.