Flying Lead from Ganesha Games. My personal favourite. No actual solo rules though, but I do believe there are solo players who use them. Very adaptable, give a good result and their site supports a 'squad builder'. Best at under 20 figures a side, ideal at about ten.
A fair few of the Two Hour Wargames rules are designed for solo play, as well as co-op or standard play. Some people think the action-reaction tests interrupt play too much, others think they are fantastic... so mileage might vary.