This is a big one as well, and one that might take us a couple of decades to achieve. Nevertheless, paying off the mortgage on our current house (or the one on our new house, in case our planned move within a couple of years comes through) is something we are going to be focusing on, now that my student loans have been paid of in full as of last December.
The main reason behind this one is that the greatest part of our monthly payments is interest (currently we even only pay interest), so the original mortgage sum has remained intact since we bought our house five years ago. Now that we have some more money available, we will try and put as much as possible towards our mortgage. It will take time, but seeing our monthly payments go down should be a nice motivator, and I can imagine that the feeling of living for free (except for maintenance and utilities, of course) will be awesome.
Currently we owe 238.500 euros, so there’s a pretty long way to go, but I will try and make a significant payment at least once a year, starting at the end of this year.