You MUST HAVE .NET 4.0 PRE-INSTALLED to install and use SQL Sentry Plan Explorer. Download .NET 4.0 here.
SQL Sentry Plan Explorer x64
SQL Sentry Plan Explorer x86
Plan Explorer Documentation (or in PDF format)
Plan Explorer Change List
The Plan Explorer SSMS add-in allows you to jump directly to Plan Explorer from SSMS by simply right-clicking an execution plan and choosing the "View with SQL Sentry Plan Explorer" context menu option. The add-in works with Management Studio for SQL Server 2005, SQL Server 2008, and SQL Server 2008 R2. Watch this space for updates regarding compatibility with SQL Server 2012.
Plan Explorer SSMS Add-In
NOTE: Plan Explorer must be installed separately – download above