There are a variety of free metrics and tools that can be used to find out lots of information about any website (whether or not you own it).
Some examples include:
Popularity as measured by Alexa (www.Alexa.com)
Popularity as measured by Compete (www.Compete.com)
Popularity as measured by Quantcast (www.Quantcast.com)
Inbound Links (most search engines can provide this and Alexa does too).
Google Page Rank (Google toolbar)
Site history from the wayback machine (www.archive.org)
Domain registration information (whois services like www.networksolutions.com/whois/)
Possible copyright violations (www.copyscape.com)
Which of these tools have you used? What do you think of them? And what other tools do you think are worth sharing with your fellow webmasters?
Michael