EP Calipers allows you to use electronic calipers to make measurements on images of ECGs or EP recordings. Use time calipers to measure intervals or heart rate, amplitude calipers to measure voltage, and angle calipers for specialized uses such as evaluating Brugada Syndrome. See this post for more details.
Note that EP Calipers for Windows is not compatible with Windows S (a high security version of Windows). The reason for this is that the download includes some open-source third party files that are required for handling of PDF files, but Windows S will not allow 3rd party files as part of the app package.
Note: some of these are a bit dated and lack some of the newer features, but they should give you a general idea of how to use the app on different platforms.
EP Calipers for Windows
(Note that this video does not demonstrate the transparent window mode)