You can send headbonks to your furriends (or anykitty else, for that matter).
Headbonks are a field in the Personal Message. Start by sending a Personal Message -- the easy way is to use the Messages link in either the upper or lower right-hand corner. Click on +Write new message, and check (tick) the headbonk field.
The headbonk will appear in the message and in the email notification.
(Think of it as the kitty version of a poke, only nicer.)