9.0 Professions
As of 4.0.6, most Professions have now been normalized to give ~80 INT which makes profession choice a bit less of a min/max decision. I'm not sure if I like that but that does seem to be the trend. That said, there are still three professions that do not fall in that mold as well as small differences in how each profession gets you to that 80 INT all of which I will describe below:
As of 4.0.6, Tailoring gives the best DPS gain.
Tailoring: Lightweave Embroidery has been updated to giv 580 Int for 15 seconds, 20% proc on spellcast, 50 second cooldown. The average benefit is around 155 Int, minus the 50 Int you usually have on your cloak, for an increase of around 105 Int.
Engineering: Synapse Springs gives 480 Int, for 10 seconds out of every 60 (as of 4.0.6 -- was 12 seconds prior), for 80 Int on average.
Leatherworking: Draconic Embossment - Intellect in place of 50 INT to bracer (available in 4.0.6) gives 80 Intellect.
Jewelcrafting: 3 Brilliant Chimera's Eye in place of 3 Brilliant Inferno Ruby gives 81 Intellect.
Blacksmithing: An extra socket each in your wrists and gloves, each with a Brilliant Inferno Ruby, gives 80 Intellect.
Alchemy: Mixology (with your Flask of the Draconic Mind or Flask of Battle) will give you 80 Intellect.
Enchanting: Enchant Ring - Intellect to each ring gives 80 Intellect.
Inscription: Felfire Inscription in place of the Therazane enchant gives 80 Intellect.
The remaining professions increase the secondary stats of Haste and Crit instead of the primary stat Intellect and provide far less benefit as a result:
Herbalism: Gives a haste cooldown via Lifeblood, it averages out to 80 haste.
Skinning[: Master of Anatomy gives 80 crit rating.