I personally do not think there's any other better cases for N900 apart from Otterbox case. I have tried those clear cases for N900 (a generic brand-less case) and they were fragile, One small drop and the screen portion of the clear case cracked.

When I purchased my 3rd N900 the seller provided two complete protective cases. Again these are not Otterbox branded case and I've found them fitting on the screen very loosely. I suspect the little plastic tabs were either broken off or the case was never developed properly. It also happens that with the provided 3rd N900 was yet another set of protective cover but only for the screen part and that was also broken.

So in a nutshell, no I've not come across any tougher case than Otterbox case myself. I have bought it at least 6 months ago and it still is going strong after a fair amount of repetitive removing and installing of the cases from the device.