Professional Software to use on Client Computers

I would like to know if there is anyone out there that has a good handle on COMMERCIAL use software that I.T. technicians can use on client computers via ‘on-site’ OR ‘remote’ system servicing. Freeware or not, I have yet to find a ‘cocktail’ of software that individual I.T. business owners can use to troubleshoot and correct issues on client computers. Seems there are plenty for ‘users’ to purchase to put on their machines, but none that can be used by remote desktop support or onsite support that I.T. pros can use. Anyone???