There are a lot of clipboard managers available, but I search one which can be used without using the mouse.
Has someone a recommendation for me?
I use Diodon (available from the Software Centre). It keeps a clipboard history of strings that you can access at any moment by pressing Ctrl+Alt+V; you can then select which entry to insert using the arrow keys. You can also change the key combination from Ctrl+Alt+V to something else.
You can also use Clipit.
You can find it into official Ubuntu repositories.
It has a combination of hotkeys to use in Preferences.
Look at the picture below -->
You can check Keepboard. It provides configurable shortcut keys for activation and other common operations.