Depends. I know
pokerstars prohibits such tools (like pokerstove). Not all sites have such restrictive policies. But, I would say that allowed or not, they aren't really ethical. The idea is that you should know the odds yourself. On the third hand, some sites even provide tools which do this (Carbon was giving away Hold'em Indicator for free, which does exactly that). I like that they at least attempted to even the field for those players who wouldn't, otherwise, know about trackers or odds calculators or how to get/install one.
At the same time, I don't know that most would be useful, unless you truly don't know what you're doing. I almost never "used" the odds component of Hold'em Indicator to guide or change my play. In any case, you benefit most from learning how to do it yourself.
*Stars allows Hold'em Indicator with a limited subset of working features. Not sure which ones, because I have never used it there.