GPaste is a clipboard management tool which comes with a command line client as well as a GNOME Shell extension. The application is very simple and doesn't support images, files and so on.
Using the configuration tool, you can set the maximum history size, the maximum element size and synchronize the clipboard with primary selection. From the extension menu you can turn the tracking on/off, which is useful if you want to copy some password, etc.
GPaste is available in the WebUpd8 GNOME 3 PPA so to install it in Ubuntu, use the commands below:
sudo add-apt-repository ppa:webupd8team/gnome3
sudo apt-get update
sudo apt-get install gnome-shell-extensions-gpaste
Update: GPaste is now also available for Unity, through an AppIndicator. To install it, after adding the PPA using the first two commands above, install "gpaste-indicator":
sudo apt-get install gpaste-indicator
If you want to only install the daemon, install "gpaste" only.
Once installed, restart GNOME Shell and activate GPaste using GNOME Tweak Tool.
Important: after upgrading Gpaste to newer versions, run the following command to activate new functionalities:
gpaste dr
For other Linux distributions, see GPaste @ GitHub.