There are hundreds of applications for OS X that place information in the menu bar. Usually, I can find one that almost does what I want, but not quite. Thankfully I found BitBar, which is an open-source project that allows you to write scripts and have their output refreshed and put on the menu bar.
I personally use it to fetch Bitcoin and Ethereum prices, but because you're limited only by what you can get Bash to output, it's extremely flexible. Even the method by which you change update frequency is elegant. By adding a refresh rate to the name of your script, the program refreshes only as often as you desire. You can see an example of my Bitcoin price-fetching script in the screenshot (Figure 1). Also notice the file format for my additional scripts (Figure 2).