Free with OS X Apple, apple.com/osx
Located in /Applications/Utilities, Activity Monitor is Apple’s all-purpose monitoring software, and it’s a very capable app that can be used to monitor CPU, memory, energy, disk and network usage. Click on a tab and you’ll see a sortable list of apps and data, along with load graphs, detailing how that aspect of your system is faring.
Because of the app’s windowed nature, it’s perhaps not as useful as an ongoing monitoring tool as the likes of iStat Menus. However, if you’re having sporadic problems with Mac responsiveness, that could indicate some strange CPU usage. Under such circumstances, you could select the CPU tab, shrink Activity Monitor to its smallest size, and drag it mostly below the screen, next to the Dock. That way you’d see the top few CPU-heavy processes at a glance, and should one repeatedly pop up and have a very high value, you’ll find a possible culprit for your Mac’s woes.