I found this on d3scene by tssguy123 but I decided to post it here in a more detailed explanation.
Needed:
Cheat Engine - You can get the latest version from their official website: http://cheatengine.org/
World of Warcraft
1. Start up WoW and log on to your character
2. Open cheat engine and target wow.exe
3. Go to www.wowhead.com and find the ID of the spell you want to remove global cooldown from (be sure to get the right rank) for example Arcane Explosion rank 10 = 42921
4. Scan for the ID of the spell
5. Target all the non-green addresses
6. Then you press the "Copy All selected items to the address list" button. (The Red Arrow.)
7. Freeze all of them, select all, and Right click> Change Record> Value. In the little box that pops up, type 28877 and save.
Worked on three servers I recently tried. Enjoy.