Is there is any site/tool that allows me to estimate an average
traffic of any particular site (not mine , so I cannot use log files)?
Something like Alexa.com... Any other resources that may show any info
about the site (like marketleap.com) would be very helpful too.

what about webalizer?  I use it and its very good to me.  
http://www.mrunix.net/webalizer/

Use http://www.alexa.com/

You can also install the google bar.  Once installed, open internet
explorer and you will find the google bar.  Among the google bar tools
you will see the Page Rank tool that will give you an estimate of the
traffic on a scale of 1 to 10,  very usefull to analyse your
competition.  However for more precise traffic numbers use the site
that I previously mentionned http://www.alexa.com.   Enter the url and
click web search.
Hope this helps,
Ric

For top website you can use
http://www.hitwise.com/

Pay service though :)