KeywordSpy spies on your competitors to get you profitable keywords (PPC and organic) that makes them rank. The tool is somewhat dated (the interface is far from modern), but still works ok: enter a domain and see the list of terms it ranks for. Or see the list of top sites that are ranking for a particular keyword. However, SEMRush seems to be doing a better job on this.

KeywordSpy’b base is huge: over 127 million keywords, enabling you to dig up large amounts of competitive data (see what your comptitors are doing and build strategies to overcome them!). Especially useful for PPC campaigns: you can see the ads affilates are using to promote their offers along with ROI metric indicating how profitable the ad is.