Start / stop init.d script without root access

I am setting up an ubuntu-server in order to host the virtuoso server.

I want to setup a simple user (without sudo) account that will (only) be able to run

/etc/init.d/virtuoso-opensource-6.1 [start|stop|restart]

(to be able to administer the virtuoso server, but not other system resources)

i guess this is similar/equivalent to any service e.g. apache2, mysql, etc

is there an option to limit access to only the virtuoso service, or an option to access to all the init.d scripts, but not the rest of the system?

You can customize which commands the user can run via sudo in /etc/sudoers (using visudo):

username: ALL = /etc/init.d/virtuoso 

Your question seems to assume that sudo is an all or nothing grant of root privilege because that is how it is often used.

With configuration you can arrange for very tight control over what sudoers can and can't do.