Google has it Pagerank (PR) rating that Google gives each page which represents the link popularity of a page. The problem with PR is that is hey only update it every 3 or so months for us to look at, so its old data (see dates of PR updates). The PR also a global link score and does not indicate the strength or link juice of the links that make up the PR. Google PR is not the most  reliable for many sites.

Microsoft have Bing’s Page Score, which is a number that they give a page. It is not clear what it is that makes up the score and Bing have not been very open with.

Rand Fiskin from SEOMOZ have introduced mozRank and mozTrust. The mozRank is similar to Google’s PR and is not affected by the penalties that Google apply, so can give you a different perspective. The mozTrust which is a measure of the domain authority of a web site. Links from authority pages are the most sort after as they pass me link juice, so this is also a useful measure to understand.