Fuze Meeting from FuzeBox also lets me share any display or application, and uses a free app for both host and audience. When I activate screen-sharing, Fuze initially shares the full desktop of my main display while hiding its own interface, so it takes a few non-obvious clicks to share an arbitrary application while still seeing my audience.
If I upload a PowerPoint presentation, I can play it right in the app, without having to share my screen. Fuze also supports uploading presentations from Keynote '09 (but not Keynote 6), although it converts Keynote files to movies before displaying them, which isn't great. Fuze is free for up to 25 participants; plans that support larger meetings start at $8 per month.
By itself, the free Google+ Hangouts can't do simultaneous video and screen-sharing from the same computer. I've worked around this by using two Macs, joining the same hangout (with the same account) on each one, using one for video while the other displays my presentation. That's a clunky workaround, but it gets the job done — and it works with Keynote.
A more elegant solution is to use the free edition of Telestream's Wirecast for YouTube, which lets you create all sorts of views within Google+ Hangouts — including side-by-side and picture-in-picture, with video of yourself and a window or monitor (for example, with a PowerPoint presentation) shown simultaneously. But, fair warning: Wirecast is not the most self-explanatory app. Expect to spend some time reading the manual and experimenting before you achieve the effects you want.
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