Casting to string:
[string]$x
vs.
"$x"
Casting a string to a numeric type:
[int]$x [float]$x
vs.
+$x
Also very useful to know that PowerShell always takes the type of the left operand to determine the final type of an expression and conversions to apply:
'1'+2 -> '12'
1+'2' -> 3
which can help determining where needless casts are.