The MSDN pages are of course the ultimate source for the .Net classes.
Which .Net classes to "learn", is of course fully dependent on the task you have at hand.
And hence it is not obvious which .Net classes to explore.
See also Don Jones thread on the subject, aptly named Why the HECK Do You Want to be Taught .NET in a PowerShell Class?!?!?!
In most cases Google (or Bing) is your friend.
Some of the classes that I used, and which turned out to be very useful (random order).
- System.Diagnostics (application debugging/diagnostics)
- System.Runtime (application management)
- System.Timers (timed events, see for example Get-EsxTop – Another Look)
- System.Text (encoding/decoding)
