In this post I will describe my setup for live online talks. For most (mathematical) talks, either online or on-site, I prefer not using TeXed slides. The main reason is that it is very hard, for me at least, to keep the pace down. Furthermore, when answering questions it is often desirable to be able to add hand-written notes anyway. Writing most of the talk by hand, I go roughly as slow as with a blackboard talk, which seems a good choice in most cases.
For taking hand-written notes on a computer, I use a Wacom Tablet (Wacom Cintiq 16 - fairly expensive and also takes quite a bit of space on my desk, but all in all I am very happy with it) and xournal++.
Another problem with replacing the blackboard by a screen is that there is less space on the screen than on blackboards: In a seminar room there is usually at least one blackboard in sight in addition to the one in current use, so that people in the audience can still see some of the previous material. One thing that helps (which I learned from Manuel Hoff) is to place the pages in xournal++ not below each other but in a horizontal line, and to change the page format so that always two pages can be shown.
It should be easy to adapt the script to other note-taking programs as long as they have an auto-save feature (or you are willing to sufficiently often save the file yourself) and it is possible to extract image files from the format in which the note is saved originally. (The only problem would be proprietary file formats that do not allow for automatic extraction of the image files. For pdf files, there are several ways to get your hand on the image files, such as pdftoppm.)