If you decide to go with a stand-alone product, make sure it will integrate or at least not conflict with your existing software products. If there is a chance you will get case or practice management software in the future, be sure what you purchase now is capable of being imported.
AnyTime Organizer
Calendar Pro
Chronilist (shareware)
CompuLaw
Rules-based legal calendar software
Contact Plus
EssentialPIM
GroupReady
Now-Up-to-Date
Mac and Windows
Time & Chaos
Voo2Doo (web-based freeware)