Command Wiki

A huge site filled with everything you ever could want to know about the commands of Minecraft! Link

McStacker

A fantastic site to help you with /summon, /give, and loads of other picky commands! Link

Minecraft Tools

This site is home can generate tellraw, title, armor color, firework, and formatted book commands with ease! Even includes saving! Link

Playsound List

TheRedEngineer has put together a huge list of all supported playsounds in Minecraft! Link

Particles List

A list of all supported Particle Effects! Link

Scoreboard Help

Cause there are SO many possible criteria in scoreboard commands! Link

Vanilla Model Maker

MrGarretto has made a crazy browser modeler that resembles Blender slightly. Pretty much make your model, pick your options, and import it! Saving is included too! Link

One Command Maker

MrGarretto also has a command generator. This can be used to make One Command creations as shown on YouTube! Link

Color Commands

ReduxRedstone made a site that has a load of useful features! I use this site to color my commands. Just paste the formatting on a "CustomName:Name" tag or such and voila! Link