Ultimate Custom Block Pack
9/5/2015: Added a more complete list of hacked blocks, including never before used boolean hacks. Finally added HSL support.
Too many blocks to list here, but these are the categories:
Motion - blocks for advanced forms of movement
Strings - all kinds of string manipulation blocks
Lists - lots of useful ways to control lists
Operators - a plethora of mathematical functions (covers Algebra, Geometry, Trigonometry, and some miscellany)
Pen - functions for rendering various things on screen
Vectors - using sets of variables and lists as vectors
Looks - simple but must-have blocks for the sprite's appearance
Sound - advanced manipulation of sound
Sensing - ways of getting specific input from the user using hacks and tricks.
Hacked Blocks - various blocks not included in plain ol' Scratch ;)