Annotation of Web content is becoming a widespread activity by which users appropriate and enrich information available on the Net. MADCOW (Multimedia Annotation of Digital Content Over the Web) is a system for annotating HTML pages that provides a uniform interactive paradigm to make annotations on text, images and video. MADCOW has also recently included novel features for groups and group annotations with reference to ontological domains, but interaction with the MADCOW client is currently limited to the use of common input devices (e.g., keyboard, mouse), requiring a precise selection of the portions to be annotated. In this paper we present a sketch-based interface, which can be used to annotate not only content but also aspects relative to the presentation of the information. While interacting with a standard Web browser, users can draw free-hand geometrical shapes (e.g., circles, rectangles, closed paths) for selecting specifc parts of the Web pages to be annotated. The interaction mode depends on the adopted input device. For example, users interacting with touch-screen devices (e.g., smartphones, tablets) can draw shapes with their fingers, but in principle any device able to detect sketching gestures can be supported (e.g., graphic tablets, optical pens). The paper discusses interaction aspects together with an overview of the system architecture. Finally, preliminary experimental tests and some considerations on the usability are also reported.