Clients 000, 001 and 066 are standard clients that are pre-delivered by SAP. These clients are not supposed to be used in development, quality and production environments.
Client 000 is basically used as working client only when you do support pack upgrade or ABAP load generations (SGEN) and implementing additional languages, etc. Otherwise, client 000 should not be used as a working client. The same applies to client 001. But the only exception with 001 is, with Solution Manager, 001 will be your working client. You will do all configurations and obtain support from SAP through this client. With other Systems like BW and CRM, this client (001) will not be a working client. Two standard users (SAP* and DDIC) are defined in the clients 000 and 001.
The client 066 is used only for EarlyWatch functions (Monitoring and Performance). The user EarlyWatch is delivered in client 066 and is protected using the password SUPPORT. This password needs to be changed for security purposes.