There are a number of companies that never show anything on their job board because they either schedule by email or through independent schedulers, so I tend not to shut anything down until the website gives me a 404 error.
Off the top of my head:
ACRA - gone but not forgotten. They were good guys
BankAtlantic - stopped doing their own shops and what shops are now done are done by other companies
CheckMark - appears to be defunct
Datatron - closed their doors owing shoppers a lot of $$. There is a special place in Hell for the likes of them.
Daymon - became Caliber
Direct Scheduling Services - from what I understand they are no longer doing shops or scheduling for others
Examine Your Practice - merged in with another company but I forget who
Granite Starr - now operating under the name Pinnacle
JC&Associates - bought out by ??? A search of the forum would show you who bought them.
Mystery Diner - I believe they are defunct
Orilio & Associates - defunct owing shoppers. See "special place in Hell"
Pacific Research Group - I thought they were defunct but it appears they still are gasping
Service Quality Solutions - became Buckalew Hospitality
SG Marketing - was swallowed by MFI and died
Shop 'n Check - was swallowed by MFI and is the Blue Portal
Tell Us about Us - is currently being swallowed by MFI and will __________________
Wireless Store Shopper - Established Technology Store Shopper and the "Wireless" site is now nonfunctional
This is not intended as a full list, but what I remember off the top of my head.