A Designated Operational Entity (DOE) is a company accredited by the CDM Executive Board that assess whether a project fulfills CDM criteria. A CDM project must be checked by two processes: Validation and Verification. Validation is done once before initial project approval. Verification is done periodically after the project has been approved or registered.
Voluntary carbon market standards allow validations and verifications by CDM accredited DOEs and/or DOEs approved by the voluntary carbon market organization. A list of accredited DOEs can be found here: http://cdm.unfccc.int/DOE/index.html