K.G.
We put our son in preschool at 2. It was great for him.. He learned to share (VERY important at that age), he socialized with other kids his age (extremely important), he learned how to sit properly in a chair when told to (for eating and doing work), he learned a lot more about religion, (he goes to a Christian school) he learned colors,shapes, sounds, etc...
Here in S. Florida, it's VERY common for parents to put their kids in preschool at 2... Even if it's just part time..
At age:
2- he went 2 full days (8:30-2:30)
3- still 2 full days
4- 3 full days
Now he's in VPK (PreK)
Our neighbor didn't put her daughter in anything, and although, her daughter is super sweet, she's very socially awkward, and still throws major fits (she's almost 5)...
I HIGHLY recommend preschool... Not fulltime, but definitely a couple days per week :)