Don't ride it! Aluminum fails catastrophically: you are running the risk of the frame shattering. The crack means that there's stress that overcame the metal. That stress is making to other parts of the frame, which now are experiencing metal fatigue and want to crack more than they did before. You run a risk of going to the ground in close proximity to jagged metal at full riding speed when you hit a pothole or something.
I understand that it's your transportation and that money's tight - I just got my transportation bike back into working order after a month of misadventures and waiting for parts, and I live on a grad student's stipend. Frames aren't all that expensive; cf. the $100 aluminum frame above. Frankly, I'd revolve a few hundred bucks of credit card debt before riding an aluminum frame with a crack in it.