This paper presents Pandora, a network monitoring platform that captures packets using purely passive techniques. Pandora addresses current needs for improving Internet middleware and infrastructure by providing both in-depth understanding of network usage and metrics to compare existing protocols. Pandora is flexible and easy to use and deploy. The elementary monitoring tasks are encapsulated as independent entities we call monitoring components. The actual packet analysis is performed by stacking the appropriate components. Pandora also preserves user privacy by allowing control of the "anonymization" policy. Finally, the evaluation we conducted shows that overheads due to Pandora's flexibility do not significantly affect performance. Pandora is fully functional and has already been used to collect Web traffic traces at INRIA Rocquencourt.