Updated 11/29: Applications are now being accepted.

Large Lots, the City of Chicago-run program that allows property owners to buy city-owned parcels on their block for $1, is now a citywide program in round 2.
The city started the program to sell city-owned land that adjacent residents were already maintaining and add the property back to the tax rolls. It is also costly to maintain them and respond to crime on them.
Residents who own property in one of the 34 community areas should review the program’s terms of application and purchase as the Chicago Department of Planning and Development has started accepting applications. Business owners and churches are not eligible.
Buying property through this program has many benefits, including purchasing it without liens or backtaxes. Residential property owners can apply to buy 2 lots for each property they own.
The Large Lots website also describes what those who purchase the lot can do with it during the first five years of ownership. (It cannot be sold during the first five years.)
You can build a new residence (single or multi-unit, depending on the zoning district), an addition to your house, a garden, or leave it as grass. Owners are responsible for paying property taxes and maintaining the yard.