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Who says you need stacks of weights to build muscle? If you like working out at home, consider the TRX Home Suspension Training Kit.
In suspension training you use your own body weight to engage more muscle groups at the same time and hit them from different angles.
I have to admit the TRX is quite expensive ($200) so if you think it's a bit too much to pay for you might want to check out the alternatives:
Also check out the 5 Toughest TRX Exercises for a Full-Body Workout.