Why stress test cpu? The answer is to test the reliability and stability and your machine/system. Running stress tests can also help you to find out whether you need to upgrade or add new cooling for your machine in the even that your system shuts down during a stress test.
I have been using Prime95 on windows machines for a while, i found that cpuburn on ubuntu perform very similar to Prime95
To install cpuburn on ubuntu
# apt-get update && apt-get install cpuburn
You can use man to see more info about cpubun
# man cpuburn
Here are some options that you can run cpuburn
burnP5 is optimized for Intel Pentium with or without MMX CPUs
burnP6 is optimized for Intel PentiumPro, Pentium II & III CPUs
burnK6 is optimized for AMD K6 CPUs
burnK7 is optimized for AMD Athlon/Duron CPUs
burnMMX tests cache/memory interfaces on all CPUs with MMX
burnBX is an alternate cache/memory test for Intel CPUs
For multiple processors/cores system: (I have a intel i3-330UM with two real cores and two Hyper-Threading cores)
So to stress my cpu i run
# burnP6 & # burnP6 & # burnP6 & # burnP6 &
I use htop to see all the cores being stressed, if you dont have htop you can install it with this command
# apt-get update && apt-get install htop
To start/run htop, you can simply use
# htop