Penny War: 

It can take place between departments at your organization.  Start by placing collection buckets in each department.  Foster competition by making pennies positive points, while silver coins and bills are negative points.  Promote sabotage as well, by encouraging competing departments to fill opponents’ buckets with bigger coins and bills.  All the while, more money is being raised.

Jeans Day for United Way:  

Designate every Friday (for example) as a casual day.  However, in order to participate, employees must donate a certain amount of money.

Bake Sale: 

Encourage everyone in the office to bring in baked goods and then hold a sale.

Guess How Many: 

Fill a jar with M&Ms, jelly beans, etc.  Whoever comes closest to the actual number of items gets to take the jar home with them.  However, each guess will require a donation.