As John_Maxwell_IV says, it really depends on the project. A good recent example is the knockout javascript library. My last job was focused on building javascript/REST-API driven dynamic admin interfaces. Without knockout, having to use just jquery & native JS I don't think it's an exaggeration to say I may have been an order of magnitude less productive. Some of the more advanced features I was able to deliver would probably have been too complex to manage.

Of course, not everybody working away at their own new great idea should expect to be able to have anywhere near that level of impact, but I think it gives a good estimate of the sort of impact that is within grasp for the most successful experiments / projects.