You can use the Get-ADUser to get a list eg. to find out all inactive user.
get-aduser -filter {lastlogondate -gt "8/1/2013"} -Properties lastlogondate | select Name,LastLogonDate | sort name
Otherwise, you can also try this active directory cleaner tool ( http://www.lepide.com/active-directory-cleaner/ ) that helps to find out all inactive user or computer accounts from AD environment and generate the comprehensive report on inactive user accounts, never logged on users and real last logon or logoff details of accounts .
You can take a look this active directory cleaner tool ( http://www.lepide.com/active-directory-cleaner/ ) that helps to find computer accounts that have been inactive for last x number of days and remove all inactive or stale user and computer accounts from active directory and transfer inactive account to another OU. It generates the complete report which are based on inactive account, never logged on, real log on/ log off accounts report and software activity report.