ReaPack: What is it?
ReaPack is what they call a package manager for REAPER. What does that mean? Well, Reaper has quite a few different ways to extend its capabilities. You can add functionality, change the way it looks, the language and more.
ReaPack is the easy way to manage that.
With ReaPack, you can add ReaScripts (such as MK Slicer), JS effects (FX made specifically for Reaper), extensions, themes, language packs, templates, web interfaces and more. I mostly use ReaPack for adding SWS extensions, and JS effects.
JS Effects
There are some really talented programmers writing FX for Reaper. JS stands for 'Jesusonic Script' -- a DSP scripting language developed by Cockos (the makers of Reaper). The plugins developed this way are sometimes called JSFX.
If you open up Reaper, click on any FX button on any track and search for JS, you'll see a bunch of plugins there already. Well, you can add more. The easiest way is through ReaPack (instructions below).
Plugin Formats: JS vs. RTAS, VST (Windows), AU (Mac) Etc.
Plugins have various formats, depending on the DAW, and OS. JS is Reaper specific. There is a way to run JS plugins on other platforms, but it's complicated.
Installing ReaPack
Follow these steps to install ReaPack:
- Download ReaPack for your OS.
- From REAPER: Options > Show REAPER resource path in explorer/finder
- Place the downloaded file in the UserPlugins subdirectory within the Resource directory
- Close Reaper and then restart it.

ReaPack Download

Show Reaper Resource Path
Browse Packages
To add packages, go to the 'Browse packages' menu. From there you can browse and install available packages,

Browse Packages Menu
Here's the 'Browse packages' window. You can search for packages in the 'filter' dialog and install them individually, or install them all. I'd suggest, at a minimum installing everything by Tukan (you can use the search box at the top) and the SWS extension.

Browse Packages