The nice thing about an on-screen keyboard is flexibility, and these tweaks help it live up to its potential (or make typing on your device better in general).
Action Menu
FREE
by Ryan Petrich
add more options to the copy/paste menu
Bloard
by GN-OS
make your keyboard light black (dark grey)
BTC Mouse & Trackpad
$4.98
by Matthias Ringwald
add Bluetooth mouse and trackpad support
Color Keyboard
$1.99
by Yoo Seung Kim
make your keyboard any color you like
iKeyWi
by Hiraku Wang
add a customizable fifth row to your keyboard
iKeyWi 2
ManualCorrect
by Aaron Lindsay (aerialx)
correct your spelling only if you tap the word
ShowCase
by Lance Fetters (ashikase)
show lowercase keys while typing lowercase
SwipeSelection
by iKy1e (Kyle Howells)
edit text faster with extra keyboard gestures
SwipeShiftCaret
by r_plus
another way to edit text faster with gestures