Is system maintenance software required? If so, what is the best? On Windows computers, people often use third party software like SystemCare in order to keep their system functioning at top performance.

Are there Mac equivalents that monitor startup, configuration and the like?  How necessary are they?  What is the best general purpose package?
Are there Mac equivalents that monitor startup, configuration and the like?


OnyX, Snow Leopard Cache Cleaner, iTweax are some options.


  How necessary are they?


Not very. Any optimization that needs to be done is already built into the Mac OS X. 


  What is the best general purpose package?


Can't help you there, I don't use such utilities.