WhoisHostingThis has a search plugin and bookmarklet to make it easy to find out what web host a site is using. Just visit a site and click the bookmarklet.
I prefer having the bookmarklet open the results on the same page instead of opening a new one. If you feel the same way, here’s the modified bookmarklet. Just drag it to your bookmarks toolbar and let’er rip.
WhoisHostingThis doesn’t always get the host correct. It appears to do a whois on the IP address at Arin, which tells you who owns the block of IPs. This works if the web host is a large one that owns its own block of IPs, but if it’s a smaller host that gets its IPs from a large organization, then it will report the site being run by the larger organization. When it doesn’t work, try Quarkbase. At the bottom right there’s a hosting section that takes a stab at it. I’ve seen several examples where Quarkbase gets it right when WhoisHostingThis doesn’t.
Domain Tools will do a whois on the domain. You can sometimes divine the host from the nameservers, but if not and there’s an IP address, click on the whois link to see who owns the block of IPs.