Space heaters are the number 1 culprit, and heat tape if you use it for your plumbing pipes. (Not including crazy displays of Christmas lights left on all night long)
My daughter decided her room was too cold for her bird and she added a space heater, which ended up costing and extra $100. per month to run it (for 3 months), it kind of ruined the "lets keep the furnace temp down to save money" thing we thought we were doing.
The heat tape for keeping the pipes from freezing added about $40.-$50 per month it seems.
When we called the electric co. that was what they told me, yours could probably help you figure it out. Leaving a clothing iron on or curling iron on all day can cost you too. Also you can switch out your incandescent bulbs for CFL in the ones which get left on/used the most. Helps a lot.
When we bought a house a long time ago, the contractor had used all 100W bulbs every where. When we changed them to CFL, we saved almost $100 from the first month's bill!!!