I believe that these tools might be a surprise even for an experienced webmaster.
Here I go (in no particular order):
dnScoop - find the value of your domains and sites. It is not always correct, but gives some kind of starting point when comparing two or more sites. Also it provides some interesting data about the domain, so you won’t need to gather it by hand.
Google Global FireFox Extension – a useful tool. You can view organic and paid Google Search results as they appear in different region/cities and more.
Joost de Valk's SEO Link Analysis is a FireFox extension that improves your backlink checking. You can see: if your link is no followed, its page rank and more.
Check Web Site's (X)HTML & CSS, Accessibility & SEO – all in on place from Webnauts.Net.