Parameter Validation

To summarize Glenn Sizemore from the Scripting Guys blog, here is how you can validate your PowerShell parameters when you write your own script (requires V2):
Param(
[ValidateSet("Department", "AppData")][string]$type,
[ValidateRange(10, 25)][int]$size,
[ValidateScript({Test-connection $_ -count 2 -quiet})][string]$computername
)

And since version 3, the PowerShell ISE is smart enough (via Intellisense) to show you your choices in a drop-down for a ValidateSet parameter.