Firefox OS might be a viable options for low end phones, and
For high end phones Ubuntu OS might be worth it as well.
Although its not necessarily something you get for free, since we havent optimised for higher end devices you can get to a point where you have too many active apps open and chew battery, there are probably other places it can improve, but it does perform nicely on higher end devices.
In any case, an open OS is a necessity and I'm glad that I didn't have to choose between iOS, Android, and WP for my first smartphone. After some major bugs have been addressed, I look forward to being able to buy a high end device on which I can install Firefox OS. I sincerely hope Mozilla continues to give the project a lot of attention and doesn't relegate it to Thunderbird status.
In particular the early 'preview' launches havent gone as I personally would hoped, less than raw code there is a level of communication and collaboration with partner launches that dont exist with say, firefox, people have been working to address these and in particular I think the reference device / flame is going to be a release that fixes a lot of this.