I've been using GotoMeeting for years for webinar training, and The RES Forum team uses Skype to talk across the oceans, but with the purchase of a Chromebook, I had to look for what else is out there for free and easy screen-sharing.
GotoMeeting/Webinar requires a download, as does Skype - and to be honest, sometimes a poor signal makes it hard for video chat, screenshare, or presentations.
Join.me is a lightweight version that is easy to pass around without multiple licenses. It's simple enough to see what's on someone's screen, and because you can select a single url, inter-company meetings held weekly all end up at the same url. A simple bookmark allows every person to logon in seconds, without all the verification, checking email, and finding passwords.
Another option is shown here - it's called Speedshare, and allows a series of users to look at the same page. I've found this very useful for demonstrations, but it's really only good for show and tell, and only applies to browsers. Another version is called Screenleap, which allows you to share one screen for an hour, up to 8 users before their pricing model kicks in.
The advantages to these smaller sites is they're lighter on the RAM and browser memory, which means that older computers and slow connections won't suffer as much from lag (pretty much the death of any conference call).
Finally - for the low, low budget version - there's always the screenshot deck. I do this for webinars, where I take multiple screenshots to show how to perform a task step-by-step, then put them in a PPT or keynote presentation (don't forget to export to PDF). This allows you to share the deck ahead of time so everyone is on the same page.