This is an ongoing list of things that one should get when taking over IT support of a client (if you can get it):
admin credentials for a domain admin account
login credentials for the router/firewall
Tech Soup login info (if applicable)
Volume licensing information from Microsoft, Symantec, or other vendors as applicable
Account number for ISP
Account number for phone carrier (if supporting phones)
Contact info for phone vendor (if supporting phones)
login credentials for registrar
login credentials for any and all DNS nameservers