How can I find what is making my Mac run hotter and causing the fans to run more often?

My Macbook Pro's fans are running louder. I've also noticed that the average internal temperature is significantly higher. (187°F as I write this.) This didn't used to happen unless I was running full-screen video or recording multitrack audio. I've made no other changes to the computer that I can remember.

Recently, I turned on Spaces, and I thought that perhaps leaving these open was running the processor more heavily, creating more heat. But turning Spaces back off didn't help.

How can I see what background processes are running? Is there a way to track this down?

You can use Activity Monitor to view all the processes running on your Mac at any time. If you prefer to use the terminal, you can use top.

iStat Pro (free)

...allows you to monitor various stats about the hardware.

You can toggle between top CPU hogging and top RAM hogging processes:

While we are talking about system process monitors like Apple Activity Monitor, let me mention that Bresink's free

Temperature Monitor

is useful to track all the various heat sensors in your system and to record graphs of readings over time. In conjunction with Activity Monitor, top or iStat Pro you can observe the correlation between active processes and CPU core temperatures.