RightField is an open source application that provides a mechanism for embedding ontology annotation support for Life Science data in Microsoft Excel spreadsheets. Individual cells, columns, or rows can be restricted to particular ranges of allowed classes or instances from chosen ontologies. Informaticians, with experience in ontologies and data annotation prepare RightField-enabled spreadsheets with embedded ontology term selection for use by a wider community of laboratory scientists. The RightField-enabled spreadsheet presents selected ontology terms to the users as a simple drop-down list, enabling scientists to consistently annotate their data without the need to understand the numerous metadata standards and ontologies available to them. The spreadsheets are self-contained and remain "vanilla" Excel so that they can be readily exchanged, processed offline and are usable by regular Excel tooling. The result is semantic annotation by stealth, with an annotation process that is less error-prone, more efficient, and more consistent with community standards. RightField has been developed and deployed for a consortium of some 300 Systems Biologists. RightField is open source under a BSD license and freely available from http://www.sysmo-db.org/RightField.