This is an area where open-source is lagging behind the commercial houses, I guess because the standard format for most of this stuff, ie Flash, is proprietary, and the Adobe-Macromedia and their partners have an advantage; that said there's some interesting work being done - a good starting point to explore the field is http://www.osflash.org/, although this is fairly geeky stuff at the moment. It's worth stepping back and reflecting on whether your objectives can't be accomplished in good old HTML, as this makes for standards-compliant, accessible content, and there are a whole host of good authoring tools around, like NVU.
Although it's perhaps not quite what you're after, skypecast might be one easy way to get the audio (and video?) conferencing aspects of Breeze for free, and VNC can do remote desktop sharing, although it's nowhere near as easy as Breeze.
If you do want to go with Flash, then the following may be worth investigating:
OpenOffice.Org has flash export from its presentation program Impress, which also has PowerPoint import; the resulting .swf are pretty basic, but it may be possible to edit these elsewhere.
Wink is a freeware Flash screenrecorder, a bit like Captivate, but it doesn't have audio support
Red5 is, I suspect, going to have a huge impact on this area eventually, although it's still in its development phase at the moment - the intention is to provide an open source equivalent to Macromedia's Flash Communication Server, which is the engine that drives Breeze. Watch that space!
Gemin-i plus is worth investigating.