Creating annotated corpora has always been one of the major challenges for grammatical error correction (GEC). Annotated corpora are critical since they enable us to evaluate the accuracy of GEC systems by providing goldstandard references. In this paper, we propose a web-based annotation tool – Writing, Annotation, and Marking Platform (WAMP) – that tackles this issue of generating annotated corpora by allowing annotators to annotate essays with ease and export the resulting annotated essays for use in GEC research. The source code and a demo video of WAMP are publicly available at: https://github.com/nusnlp/wamp