SigViewer is an open source cross-platform biosignal viewer designed to visualize and annotate biomedical data streams. It supports a wide variety of file formats, including BDF, EDF, GDF, CNT, BrainVision, and BCI2000. Recently, support for loading multi- stream XDF data has been added. Besides visualizing raw data, SigViewer supports loading, displaying, creating, and editing events that can be used to annotate specific segments within a signal. Other useful tools include offset removal, computation of event-related potentials, and calculation of power spectral densities. To our knowledge, SigViewer is the only open source cross-platform multi-format biosignal viewer currently available that supports XDF files. Furthermore, SigViewer is completely free in that it does not depend on any proprietary software such as e. g. MATLAB. SigViewer is actively maintained and widely used across the globe (as measured by the monthly downloads). Filtering data in the frequency domain before visualization to e. g. remove line noise or excessive drift is one of the next planned features for a future release.