Donjon – RPG-related text generators
Seventh Sanctum – broad variety of text generators
Chaotic Shiny – RPG and writing related text generators
The Generator Blog – a blog about content generators
Random Recipe Generator – recipes with ingredients and instructions
Serendipity – names of people and places
Springhole – broad variety of text generators
The Forge – a good-looking collection of name generators (requires Flash)
Fantasy Name Generator – names of various types (e.g. Greek names)
Randomized Demons – descriptions of demons
Phrase Generator – extraordinary phrases and sentences
Game Idea Generator – phrases to use as inspiration for gameplay
Name Generators – names for inns, villages, food, weapons and NPCs (Flash required)
Sentence Generator – an inspirational sentence generator (Flash required)
PlotNarrator – short fairytale generator

Music & Sounds

Autotracker – Pythonscript, creating procedural songs in .au format (can be converted using VLC media player)
Abundant Music – creates procedural songs online
as3sfxr – creates retro-sounds (requires Flash)

3D Models

Arbaro – creates trees
Ivy Generator – grows ivy on 3D Models
Structure Synth – abstract 3D structures

2D Graphics

Fireball Explosions – explosion textures
Running Brushes – “random”, seamless textures
NeoTextureEdit – creates seamless textures by combining procedural base patterns, like perlin noise
Sprite Generator – abstract sprites
Turtle Toy – a HTML5 framework for creating 2D content generators
CanTree - tree sprites

Resource sites

Blendswap – Blender models one can use for animations or games after registering
OpenGameArt – a lot of useful resources for games
freetems – a lot of CC0 resources
ArtSader – graphics and music resources
BurningWell – public domain photographs
Wikimedia Commons – all manner of content under permissive licenses
Dafont – free to use fonts
OpenClipArt – CC0 .svg graphics
FreeSound – sound effects
PDSounds – public domain sounds