Link Popularity is a measure of the number of inbound links to your website. It is becoming increasing crucial as more of the search engines are using this assessment when determining rank and positioning. The most well-known of the search engines to incorporat link popularity is Google quickly followed by MSN.com
There are some very good articles written about the importance of this strategy at 101 Linking.
Link
Popularity 
This free service allows you to query Google, Altavista, and
Hotbot and reports on link popularity.
SEO
Link Popularity
This tool will query all the major search engines (Google,
Yahoo, MSN, and Teoma) when you enter the 'link:' operator
with your chosen URL. It will return the total link count
for each URL.
SEO
Site Link Analyzer 
This tool will analyze a given web-page and return a table
of data containing columns of outbound links, and their associated
anchor text.
Mike's
Link Popularity Checker
Link Popularity Checker offers instant, online reports of
a webpage's link popularity rating in 4 major search engines;
Google, Yahoo! Search, MSN, and Teoma (Ask Jeeves).