For PowerShell 3.0 and greater:
Get-ChildItem -Directory
You can also use the aliases dir, ls, and gci
For PowerShell versions less than 3.0:
The FileInfo object returned by Get-ChildItem has a “base” property, PSIsContainer. You want to select only those items.
Get-ChildItem -Recurse | ?{ $_.PSIsContainer }
If you want the raw string names of the directories, you can do
Get-ChildItem -Recurse | ?{ $_.PSIsContainer } | Select-Object FullName