TeamViewer is an excellent screen-sharing and file-transfer app that can be used to facilitate business collaborations, remotely access a second computer, or help troubled relatives diagnose and cure computer problems. Along with being free for noncorporate use, it gives users precisely the tools they need to share screens securely, send files with a minimum of hassle, control access rights, and even flip which user has control.
Now Teamviewer is available for Linux too. Still in beta stage but includes the useful features.
Some of the more recent additions -- like per-application screen sharing via TeamViewer's toolbar button -- haven't made it into the Linux version yet, but things administrators need most are there. Remote (both hosting and control), file transfer, chat (both text and voice), and session recording all work nicely. Partner Lists are also available on Linux, making remote connections to your saved hosts a cinch.