You could configure sudo to let the user in question run this program without using the sudo password.
Install sudo if it's not on our system already. Use the visudo program to edit the sudo configuration and add a line something like this:
# where "matthew" is my username
matthew ALL = NOPASSWD: /usr/sbin/hddtemp
If it's a whole bunch of users you could create a new group and then allow members of that group to execute "sudo hddtemp" without a password, for example:
# Assuming you created an "hwmon" group, and added
# all the users you want to access hddtemp to it...
%hwmon ALL = NOPASSWD: /usr/sbin/hddtemp
By the way, I'm assuming the hddtemp program is safe. The specified user(s) will be able to launch this process with root prividiliges, so, as with any program you grant permissions like this to, you should be careful to check that it can't be used to get a shell or do other nasty things.
For more information on the format of the config file, see the sudoers(5) manpage.