I'll assume you meant waiver wire since you said 'pick ups'.
1. Brian Westbrook: There could be some juice left in those legs since he really hasn't played much at all this season. I see no reason why he can't make it through the remaining schedule. Yes, he is old. He has had some injury issues with the knees in the past. But for the most part he has been pretty tough throughout his career so I expect him to be the man in San Francisco for the rest of the season. Of course Dixon will get some work as well, and possibly goal line carries. Still, opportunity plus talent. If you have a blind bidding system he will probably be the hot item this week so he may be tough to get.
2. Maurice Morris: I don't think Best is going to be making much of an impact for the remainder of the season. He hit the rookie wall way back in week 2. It has come to the point where he is hardly seeing the field. Enter Maurice Morris. He has been productive in his career before as the backup running back forced into duty (in Seattle). He is a journeyman back up running back and nothing more, but the opportunity exists. He should be involved in the passing game. I don't know that you can count on multiple TD games out of him, but he should be good for decent yardage and around 15 touches per game. And, lets face it, there isn't much better on the WW at this point (depending on the league).
3. Anthony Dixon: He is worth a flyer as well. If anything were to happen to Westbrook he would be forced into the featured role. Even if not, he should be good for maybe 10 - 15 carries per game and possibly get whatever goal line looks San Francisco is able to create. I wouldn't want to start him against the Chargers (week 15) unless Westbrook is out. But he could be a decent start if you are desperate against the Seahawks (week 14) and the Rams (week 16). These are teams that San Francisco should be competitive with, creating opportunity for Dixon even if Westbrook stays healthy.
Other flyers:
In a more shallow league there may be guys like:
Addai: could end up a great start in Week 16. I wouldn't start him week 16 unless he plays before hand and appears to be back to normal.
Ivory: seems relegated to more goal line duty than anything, but the Saints are a high powered offense so there could be some opportunity there.
If you are very desperate:
Julius Jones, Tashard Choice