This is an interesting website about carseat safety that gives recommendations for what to do in taxis.
http://www.thecarseatlady.com/taxis/using_taxis_5.html
Probably the most widely available of what she recommends is the backless booster seat which is available at many stores.
Of what she recommends, I have the safeguard go -
http://www.safeguardseat.com/go/index.htm - first few years at paragon sells it. It can be used as a booster seat above 40 lbs/18kg and it folds into a carry bag. It transforms from a backless booster into a booster with a five point harness if you have latch with a back tether, which is what we use in the grandparents car (or if you travel, a rental).
I also have the Cosco Safety 1st Tote N Go which I got at a yard sale. I don't think is sold any more but at least one of the toy rental places rents it. She doesn't really recommend it because she finds it hard to get it to fit well. I don't use taxis very frequently so until I got the Safeguard Go, I just made use of this. I figured it was better than nothing.
She recommends the Ride Safer Travel Vest - it looks a lot more sturdy and secure than the Tote N Go and if I took cabs more frequently than I do, I'd probably strongly consider buying it. Unfortunately I don't think its available at any stores here that I have seen - you may have to buy it online.