Join.Me is a live screen sharing program, so if you want to show someone how to use a software program, share a PowerPoint presentation, review edits to a document, run a webinar, etc., you can do it all (and share with up to 250 people) by clicking to share your screen and providing your audience with the number code you're given. Viewers simply go to the Join.Me website, enter the code, and view exactly what's happening on your screen.
Join.Me also allows you to share files with viewers, chat, and enable audio during the presentation. You can get downloads to view on mobiles, as well.
Join.Me does have paid Pro options for business as well (yep, it's a freemium!), but if you don't need to switch presenters, have a meeting scheduler or international conference lines, you'll probably be fine with the free version.
Check it out and let me know what you think!