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