Ask HN: What tools do you use to improve your presentation skills?

Are there any online tools or portals you would recommend to improve someone's presentation skills? Any specific courses? And what are their drawbacks?

Context: I often see developers who are extremely skilled give poor presentations. These performances seem to ruin an otherwise stellar content. Although there are many companies who do offline (in person, 1 to n) training, it would be great to have your input on tools which achieve the same or better, but online. Thanks!