Join.me requires a small installation to share your screen with others. However, it also allows chat, voice chat, and sharing control of your screen with others.
Screenleap.com uses a Java applet to share your screen, so no installation is required. You cannot, however, share control of your screen. However, if you are suggesting that someone share their screen with you, this software requires no download on their end.
A third alternative is Teamviewer, which is also free for personal and non-commercial use. Teamviewer installs on your computer, and it can be set up to run at startup of a remote computer (nicely automated if helping a tech-challenged friend or family member, but which would likely require setup assistance).
For screen sharing of mobile and tablet devices, try TeamViewer QuickSupport; the platform allows a screen shot of the mobile device and a posting of the screen shot on the remote computer. (Dynamic screen sharing of mobile devices is currently not available – let us know if you hear of something though!)