How to count objects in PowerShell?

This will get you count:

get-alias | measure

You can work with the result as with object:

$m = get-alias | measure
$m.Count

And if you would like to have aliases in some variable also, you can use Tee-Object:

$m = get-alias | tee -Variable aliases | measure
$m.Count
$aliases

Some more info on Measure-Object cmdlet is on Technet.

Do not confuse it with Measure-Command cmdlet which is for time measuring. (again on Technet)

Just use parenthesis and 'count'. This applies to Powershell v3

(get-alias).count

As short as @jumbo's answer is :-) you can do it even more tersely. This just returns the Count property of the array returned by the antecedent sub-expression:

@(Get-Alias).Count

A couple points to note:

1. You can put an arbitrarily complex expression in place of Get-Alias, for example:

   @(Get-Process | ? { $_.ProcessName -eq "svchost" }).Count
   

2. The initial at-sign (@) is necessary for a robust solution. As long as the answer is two or greater you will get an equivalent answer with or without the @, but when the answer is zero or one you will get no output unless you have the @ sign! (It forces the Count property to exist by forcing the output to be an array.)

2012.01.30 Update

The above is true for PowerShell V2. One of the new features of PowerShell V3 is that you do have a Count property even for singletons, so the at-sign becomes unimportant for this scenario.