Google has offered a "Similar" feature on results for a while now as a way to discover new, useful sites, but it was easy to overlook. Now they are showing these alternative sites more prominently. For queries where similar sites are likely to be helpful, Google has started to display a list of "Pages similar" at the bottom of the results page. For example, if you search for Rootsweb, this is the list of sites similar:
Pages similar to http://www.rootsweb.ancestry.com/:
- FamilySearch.org - Search for family ancestors. Billions of free family ... - familysearch.org
- USGenWeb Project - Consisting of a group of volunteers working ... - usgenweb.org
- Genealogy Today - genealogy and family tree history, ancestry ... - genealogytoday.com
- genealogy, ancestry, family history - Discover Your Ancestry, ... - kindredkonnections.com
And right below that is Additional Searches Related to Rootsweb.
While a broad search like Rootsweb might give very obvious similar sites, try checking out Similar Sites when searching for surnames and localities related to your family tree and see if this the newest Google search feature is useful to you.