Some men just want to watch their processor core burn.
Maybe you need to benchmark your system, check if the alarm you set up actually triggers, or simply want to heat your lap on a cold rainy day. This is a quick & dirty trick I use:
(xz -6c /dev/urandom > /dev/null) &
Run twice to overload two cores.
kill %1 %2 ... to beg mercy.
Each process consumes ~100MB of RAM, so running enough xz processes will also check how reliable your RAM is.
How/why does this work?
xz implements a very CPU-intensive LZMA2 compression algorithm.
Feeding it with a random byte stream simulates the worst case data, driving L1/L2 caches insane.