General boredom:
* Tedium
* Monotony
* Dullness
* Ennui
* Lethargy
* Listlessness
* Apathy
* Weariness
* Lassitude
* Languor
Boredom with a sense of restlessness:
* Restlessness
* Fidgeting
* Impatience
* Discontent
* Frustration
Boredom with a sense of negativity:
* Despondency
* Melancholy
* Gloom
* Dejection
Boredom with a sense of emptiness:
* Vacuity
* Emptiness
* Meaninglessness
Informal words:
* Blah
* Drag
* Snoozefest
* Yawnfest
The best word to use depends on the context and the specific feeling you want to express.