The problem with any general web monitoring software which has publicly available results is that it is open to abuse. Alexa is a browser plugin owned by Amazon.com which detects user traffic and ranks the web on it.

The problems with Alexa are that
1.) it has an incredibly small user base.
2.) it is very easy to manipulate.

http://www.sitereturn.com/ is a network which places computers on autopilot to search other peoples websites through alexa while you are away. In return you get half your visits back by others linked into the same ring.

Its fun to read terms of services from such networks as they try to appear ethical and as if they have standards. "Should you wish to promote a site that is adult in nature, we suggest creating neutral pages for the sole purpose of boosting Alexa ranking." - good stuff