Here are the ones I use on a regular basis, at least the ones I can remember right now.
Not listed in order of importance, just listed as I think of them.
NamePros (Of Course!)
G-whois
Domaining/com
InternetArchive
Namebio
ExpiredDomains/net
MailTester
ZFBot
Ahrefs
InfoZoom
ZoomInfo
CrunchBase
TMDN.org
DPMA.de
IPO.gov.uk
DomainIQ
EmailFormat
TldList
GoogleTranslate
DomainTools
USPTO/gov
DN Journal
GDAuctions
Namejet
Snapnames
Dropcatch
GoogleTrends
DomainSherpa
Spyfu
There are a few more, I am sure.
If I remember, I will add, if I can