Mac: You have a ton of options for different system monitors on the Mac, but if you're looking for something a little more customisable than the rest, MenuBar Stats 2 is worth a look.
Like any system monitor, MenuBar Stats can show CPU details, memory info, disk usage, networking and battery life. More interesting is the plugin support. With plugins, you can add your own modules, like temperature or fan speed. Whether more plugins come around is up to developers, but it's nice the support is there. If you want to give MenuBar Stats a test run, you can check out the demo on the developer's site.