Windows Task Manager equivalent for Mac OS X?

Activity Monitor. It's included with OS X. Just look in your Applications/Utilities folder or use Spotlight to find and open it.

Activity Monitor found in Applications/Utilities folder.

O'Reilly Mac Dev Centre has a good run down on how to "read it". And Peek-a-boo is a good way to show you how processes connect to one another.

Although if you really want to know what your system's up to, I'd recommend iStat pro.

As others have answered, Activity Monitor in /Applications/Utilities/ is the most direct equivalent to the Windows Task Manager, but there are other options..

I mainly use iStat Menus to work out which application is using up all the CPU time..

Instead of having to launch Activity Monitor (which takes a few seconds to start), I just click the little CPU menu bar item, and it lists the current top processes:

You can also use the "top" command instead of Activity Montior (it's quicker to launch also). Just run the top command in a terminal:

I have an alias ltop which launches top, ordered by CPU usage, with a few flags to reduce it's CPU usage (reduces it's accuracy with regards to memory usage, but uses about 2% CPU instead of about 10-12% by default):

alias ltop='top -F -R -t -o cpu'