Intuit Online Payroll is very straightforward and fairly user friendly. Be aware it is only viable for single Local withholding situations. Cost is less than $20 per client monthly with additional per employee / contractor / multiple state fees. Quarterly Payroll Tax and Annual Reconciliation is a breeze!
Asterisk - if you allow clients to self process / edit payroll on the fly - as with everything the more client participation the more housecleaning falls to you.
Intuit offers several payroll options. Different brands different abilities. This recommendation is specific to Intuit Online Payroll formerly Pay Cycle https://iop.intuit.com/login/aclogin.jsp