While compiling my Top 10 Reformed Villains, there were some characters that could have made the list, but I left them out for specific reasons. I suppose you could say they are honorable mentions, but to my mind they just didn't quite fit the criteria.
Sandman - I desperately wanted to include the Sandman on my list, as I was quite fond of him as a hero/mercenary with the Wild Pack. I particularly liked his extremely brief stint as an Avenger, where he called in the team to deal with an arms shipment and Captain America told him it was kind of beneath what they usually did. Sandman had a fit, and I think it was justified.
Wow, Marvel has a lot more reformed villains than DC...other than maybe Plastic Man and Major Disaster I'm having trouble thinking of any, especially with the same prominence as the Marvel characters.