ClipShare allows you to share clipboards between two computers with GTK installed. Requires a recent version of perl, perl-Gtk and perl-Glib.
ClipShare can send more than just text - you can copy and paste pictures and most other complex data formats.
On the server (must expose TCP port 6969):
On the client:
./clipshare <server hostname>
Date is transferred unencrypted, so the use of eg. an SSH tunnel is recommended for transfer across public networks.