HosterStats.com has two great features for domainers:
1) Domain History
You can give HosterStats a domain name, and HosterStats will show you history of DNS changes for free. This is great because to check the registration history of a domain.
For example in the screenshot below, you can see the domain history for my domain, PrestaValve.com. I hand registered PrestaValve.com in March of 2009. It had been previously owned and dropped. The statistics database goes back to 2000.
For domainers, this is valuable information. It tells you how often a domain has been registered in the past (which is related to its age), plus you can tell how many other people tried and failed with that domain already.
Compare the above to the DomainTools History Tool (screenshot below), and you'll see that you get much more free information (without the DomainTools membership):
2) Host Statistics
You can give HosterStats the name of a domain host, and it can tell you how many domains are hosted there. Not only that, but you can see previous years to look for trends with your domain host.
So far example, the PrestaValve.com domain is hosted on DevHub, where the host is called KAPPACORE.COM (see above screenshot). If I enter KAPPACORE.COM into HosterStats I get this screen:
This tells me that DevHub is hosting about 1400 sites (at least on that one DNS server) and that it's trending down (it was 1659 last year). Not sure how accurate that is, but it's one way to find out how many sites your webhost has.
Happy Domain Hunting!