Chrome: Previously mentioned screen-sharing service Screenleap made waves because it allows you to share screens with someone else with a single-click, no plug-ins or client installs required. Now, thanks to the new Screenleap Chrome Extension, you can launch screen-sharing sessions from inside Gmail, using your Google Contacts.
You can still go directly to Screenleap and start a screen sharing session, but the new Chrome extension allows you to launch one from inside Gmail at any time, without installing a client or a plug-in to make the session work. You'll notice a new "Share Screen" button right under the "Compose" button, and the plug-in will pre-populate an email with a link to connect to your session that you can fire off to your contacts. If the person you want to share screens with is already a Gmail user and is logged in, you can click "Share Screen" while viewing their contact to launch a session and invite them to it directly via GChat.
Screenleap and its Chrome extension are both free, and the extension is available now in the Chrome Web Store. If you want quick screen sharing, you have plenty of options: you could use Google+, or use another popular service like LogMeIn, TeamViewer, GoToMyPC or Join.Me. The nice thing about Screenleap is that it doesn't require installations on either end to work.
Screenleap for Gmail [Chrome Web Store]