I am importing several thousand Leads at a time, and unfortunately the quality of the Contacts data already in Salesforce, as well as the quality of the Leads data also in Salesforce, makes it very difficult to do the Lead to Contact conversion quickly. So many of them are potentially duplicates. Is there a tool which can compare more than just name and email, as the LEad to Contact converter offers? E.g., check for matches in a number of fields such as comparing the Lead phone number with the Phone, Mobile Phone, Work Phone, and Other Phone. Or look for a match between Lead Address both Contact Mailing address and Contact Other address. Or all of the above.

Looking at my org's Company Profile screen, I see that I have 25 "Guest User License" licenses. 

What exactly does this limit me to?  I see that 2 are used and 23 are not.  I do have 2 sites, so does that mean that I am limited to 25 sites?  Is it 1 "Guest User License" per Site?

When I saw these licenses, I started thinking these 25 licenses meant that I can only have 25 concurrent Guest users accessing my Sites, but hopefully that is wrong and that the license is just 1 license per Site.

Is there anything limiting how many concurrent user can access the public side of a site?