As an outsider to the SkyMiles program, it seems to me that the best yield one could get on one's miles is the discounted business class (or domestic first class) award tickets. I wish there was an easy way to search for those.
http://AwardNexus.com will also show DL award inventory (also screen scraped from the Delta.com award search) however unlike other tools it has a free version for FT members and no download required.