Tinker's Kit

Tinker's Kit is sold by Jeeves, Scrapbot, all Engineering Vendors and at your Engineering Building in your Garrison at only 2 Gold each. It can also be gifted by Blingtron.
You can also craft them yourself, but please you shouldn't. Unless you skill up your engineering with it (Skill 550 to 580), or else it is too expensive compared to 2 gold each at vendors. So dont' craft Tinker's Kit yourself!

Macros for smart & convenient activation of engineering tinkers:
For Cloak-Tinkers: Golblin Glider, Flexweave Underlay do this:

#showtooltip
/use 15


For Belt-Tinkers like Nitro Boosts, Watergliding Jets, Frag Belt, etc. do this:

#showtooltip
/use 6


Why should you use these macros?
This way your macro will picture your belt or cloak. And it will still work, if you change your item to another cloak or belt, which has been tinkered! Bind these macros to a key and enjoy to the maximum!

UPDATE:

OLD!
Speaking as of Patch 6.1 tinkers on gloves have been removed, before it was:
Gloves-Tinker: Synapse Springs (Mark II), Incendiary Fireworks Launcher, etc

#showtooltip
/use 10


Used for all Tinkers Past and Present as of Pandaria

Goblin ^&*!@# - Goblin Glider
Permanently attaches a specialized folding assembly to a cloak, allowing you to deploy a goblin @#$%^& and fall slowly for 30 sec. The *!@#$% has some initial thrust and allows you to turn while falling.

Incendiary Fireworks Launcher - [spell=109077]
Permanently attaches a mounted fireworks launcher to your gloves, allowing a skilled engineer to deal (35 * 7) Fire damage over 3 sec to an enemy at long range. The launcher can only be fired once every 45 sec.

Phase Fingers - [spell=108789]
Permanently attaches lightweight transfluxers onto gloves, allowing you to partially phase and increase dodge by 20520 for 10 sec. Gloves can only be activated once every minute.

Synapse Springs Mark II - [spell=126731]
Permanently attaches synapse springs to your gloves, allowing you to increase your highest primary statistic by 1920 for 10 sec. The gloves can be activated every minute.

Watergliding Jets - Watergliding Jets
Permanently attaches waterjets to your belt, allowing you to walk on water and swim quickly for up to 6 sec.

also sold by engineering trainer