This is a list of open source software for presentations. Yes, there are a lot of other applications out there. This is what I use.
GUI tool. Good for those who aren't distracted by pointyclickybuttons.
Markup language based on DocBook XML. Good for markup gurus. I will typically use this and export HTML pages. When I'm teaching web development classes I prepare my overheads directly in HTML.
TeX (ish) markup language. Good for people that are distracted by pointyclickybuttons but who aren't markup gurus.
There are tens if not a hundred different tools out there. Some are good, some are not. Try a few to see what you like.