Using Activity Monitor in SQL Server Management Studio
Monitor Microsoft SQL Server™ for higher performance and availability with WildMetrix™
SQL Server Performance Monitoring Tool - Free SQL Monitor
A DBA guide to SQL Server performance troubleshooting – Part 2 – Monitoring utilities
If you want the market-leading SQL Server performance monitoring tool, that includes 24x7 monitoring, alerting and diagnostics, try Idera… | Sql, Sql server, Server