ADB and Fastboot are two of the most important utilities when it comes to Android development or modding. It used to be a pain to install them without downloading the entire Android SDK, but ever since Google started publishing the SDK Platform Tools package separately, it's not too difficult to install ADB and related utilities within a few minutes. However, there is an even easier way to get everything installed on whatever computers you have: Nexus Tools.
Nexus Tools was first created by me in 2013 as a one-click install method for ADB and Fastboot, before Google offered a separate download, and it was actually covered by XDA in 2014. You just type in one command in the Terminal (or PowerShell, on Windows), and Nexus Tools will download the latest SDK Platform Tools package to your home folder and add everything to your system's path variable. After that, you can run ADB, Fastboot, and other utilities as normal.
Nexus Tools 5.0 was just completed, which has a rewritten codebase (it's now a command-line Dart application) and initial Windows support. The new Windows version works just like the Linux/macOS version, but it can also download and install ADB drivers for you, which aren't required on other platforms. Nexus Tools also now installs itself to the same directory as the SDK tools, so you can update ADB and Fastboot at any time by running nexustools -i.
It's not too hard to install Google's Platform Tools package yourself these days, but Nexus Tools can still save you a few minutes of time, and it provides easy updating. The code is also open-source on GitHub.