The combination of this plugin and the Smart Donations plugin allows you to create very flexible contribution forms. As far as I can tell, it's the best affordable solution out there. The free versions have some limitations, but the paid versions are inexpensive and worth it.
The support is fantastic. Edgar is committed to fixing problems users identify, and he communicates with you promptly. I hope more users discover this plugin and Edgar makes some money off of it. He deserves it!