Tools
RetroAchievements
Lets you log in and configure RetroAchievements. See this page for more information.
Savedata Manager
Lets you view and delete savedata.
System Information
Shows device-related information such as the system specs, OpenGL and Vulkan extensions and features, GPU driver versions, detected CPU cores etc.
Developer Tools
A large amount of obscure settings and test screens that have been added over the years as needed to help development.
Remote Disc Streaming
Lets you play ISOs shared from another instance of PPSSPP running on the same network. Can be convenient for testing many games quickly on a mobile phone for example, without having to copy the games over to the device, which might fill it up unless you carefully manage storage.