Project Management
Under the File menu of Reaper, there are a few options:

"Save list of open projects..." creates a file that lists all of the open project tabs. The list file is saved with a .RPL (Reaper Project List) extension. This file format is just plain text with one line per Reaper project filename.
Selecting "Open projects from list..." opens the .RPL file, and then opens all the projects listed in it. Optionally you can replace all open tabs with the projects in the .RPL list file.
This is useful if you have an album's worth of tracks and you want to store a "session" such that it's easy to open all of the tracks at once.
You can also save "related projects", or built in links to other project files. The links are stored in the project file so you only need to create the link once. To create the link, select "Add related project..." and browse to your related project.
This feature is useful if you have some tracks you want to archive: Create a project named "archive.rpp", then cut and paste your unused tracks into this project. Set up the link to the related project so the archive project is always a keystroke away. In fact, a macro to do this automatically is very easy:
- Tracks: Cut selected Tracks
- SWS: Open related project 1
- Item: Paste items/tracks
- File -> Save project
- Close current project tab
(Thanks to Mercado_Negro for the idea!)
There's also a Project List dialog, from File->SWS Project Management->SWS Project List.

Actions
As always, there are actions available for keybinding and macro building:
- SWS: Save list of open projects
- SWS: Open projects from list
- SWS: Add related project
- SWS: Open related project 1
- SWS: Open related project 2
- SWS: Open related project 3
- SWS: Open related project 4
- SWS: Open related project 5
- SWS: Switch to first project tab
- SWS: Switch to last project tab
This area of the extension is new, please let me know of any issues or suggestions.