Our security team complained to me that they found a lot of users with trivial passwords simply by trying to log in as them.
They asked me to write them a script to speed up the process, so I wrote them my brute force guessing script.
It's not very stealthy, as it's doing multiple authentication attempts against the Domain Controller (and unlocks the user in between).
It's basically designed for domain admins wishing to search for one or two particular passwords on their domain without bothering to analyze the ntds.dit file.
It can optionally re-lock users it unlocked during testing (if you don't want to disturb anything...)
The parameters are simple - the users and passwords you wish to test and the relock parameters (whether to relock and how many times a wrong password should be attempted to lock a user).

param(
    [string[]$users,
    [string[]]$passwords=('Password1','Password2'),
    [bool]$reLock=$true,
    [int]$lockRepeats = 3
)

ipmo active*

function test-UserPassword($userName,$Password){
    $locked = Get-ADUser $userName -prop LockedOut -errorAction 'stop'| select -exp LockedOut
    if($locked) {Unlock-AdAccount $userName}
    ([bool](new-object directoryServices.DirectoryEntry '',$userName,$password).Name)
}

$users | Get-ADUser -prop LockedOut,canonicalName -ErrorAction Continue | %{
    $userName = $_.samAccountName
    Write-Debug "Working on user $userName"
    $matchPass=$null

    for($i=0;($i -lt $passwords.Count) -and ($matchPass -eq $null);$i++) {
        $password = $passwords[$i];
        Write-Debug "Testing user $userName : $password"
        $passOk  test-UserPassword $userName $password
        if($passOk) {
            Write-Debug "Found $userName : $password"
            $matchPass=$password
        }
    }


    if($reLock -and $_.LockedOut) {
        # Relock the user
        for($i=0;$i -lt $lockRepeats*2;$i++){test-UserPassword $userName $i | out-null}
    }
    $_ | select SamAccountName,canonicalName,@{name='OriginallyLocked';Expression={$_.LockedOut}},@{name='MatchingPassword';Expression={$matchPass}}
}

Enjoy your brute forcing!