For around a hundred dollars and few hours of time, Instructable user HHarry has created a wooden rowing machine to help strengthen his muscles without significantly weakening his wallet.
The rowing machine might not be the most popular or expensive exercise device out there, but building your own will give you a chance to customise the look and feel of the equipment. It's a great machine to add to your core workout because it works your arms, legs, stomach and back while causing less stress on your joints. Hit the link for the full set of instructions.
DIY Rowing Machine [Instructables]