I know there are much more fancy tools you can buy on the market than the standaard build-in Activity Monitor from SQL Server, but is a usefull tool that can help you monitor your Server. You can find it when you right-click on the server instance and then click on Activity Monitor.

The 2 grids I watch most is:

• the processes grid (instead of sp_who2) -> for example to detect/remove DB-locks
  
• Recent Expensive Queries -> to improve my indexes

A other tool (but payed) that can give you very good information about your SQL Sever is ‘SQL Sentry‘. You can download a trial from their website.