There are many cool pre-defined constants, functions, methods, and etc. in System object. One would normally learn individual ones through sample scripts, but really should browse the MSDN page to explore what System namespace has to offer. Go to a class/structure/enumeration that you are interested, then pay special attention to those static members.
A few examples:
[System.DateTime]::Today versus [System.DateTime]::Now
[System.String]::Empty
[System.Console]:: almost everything are static, not surprisingly
[System.Math]::PI
There are also a bunch of other namespaces, please see .Net Framework Libraries