Depression is one of the conditions that always runs alongside ADHD - even if you don't fully qualify to wear the official depression badge. The other is anxiety.
I have listed most of the conditions that commonly co-occur with ADHD here. You will find stories about these conditions in this section. The conditions are listed in no particualr order.
Bi-polar and it's different flavours, Dysthemia and Cyclothymia, and the appropriately named SAD - Seasonal Affective Disorder.
Two conditions that occur on their own but are often severely aggravated by ADHD symptoms are the monthly Premenstrual Syndrome or PMS. This occurs only in women
The other is postpartum depression, that (usually) 6 week-long period after the birth of a child. There are other conditions that can occur, but these are the ones that most impact on ADDers, and are impacted by ADHD
For context and an overiview of ADHD and Co-Occuring Conditions click here