Here's a list of apps that are compatible with the Multi Window feature for non-rooted devices & non-modified apps. Please help make this list as complete as possible. If you know of any that aren't on the list, please post them (preferably with links) & I'll add them to the list.
Below the compatible apps list is another list of apps that can further make use of the Multi Window feature. They likely won't be in the panel.
For those new to the Multi Window feature & want a quick guide on how to use it, scroll to the bottom of this post.
Multi Window Compatible Apps
135
Multi Window Miscellaneous Apps
3
Multi Window Quick User Guide
NOTE: Some of the steps may vary from device to device & from carrier to carrier.
Turn On Multi Window
There are 2 ways to completely turn on (or off) this feature. 1) Go into your Notification Tray > tap the Multi Window toggle (lit = on, unlit = off). 2) Tap Menu > Settings > Display > put a check in the Multi-window box to turn on (or uncheck to turn off).
Activate, Edit & Adjust Multi Window
To activate (or deactivate) Multi Window, touch & hold Back. A light blue tab &/or the Multi Window panel will appear (or disappear).
To edit the Multi Window panel to show only the applications you want, with the panel visible, touch the Edit button, then drag & drop
apps from the panel to the screen or vice versa. When finished, touch Done. To change the order of the
apps within the panel, drag & drop them where you want them. This may be done with or without the Edit button being activated.
To adjust the placement of the Multi Window panel, with the panel visible, touch & hold the tab & drag it to any edge of the screen you like. To adjust the placement of the Multi Window tab, with the panel closed, touch & hold the tab & slide it to the position you like.
Use Multi Window
To start Multi Window, touch an app from the Multi Window panel. This opens the first app in the Multi Window screen. Touch & drag another app to the top or bottom of the Multi Window screen to add the second app. The viewable area of each app may be adjusted by dragging the separator bar between the 2
apps. Touch the separator bar to activate the icons to Swap

the windows or make one of the
apps Full Screen

.