PomoDone is definitely the best Pomodoro technique app around there. I've been using it since the first versions, a couple of years ago, and had tried several other options before.
Two points I can highlight are:
1. The ability to sync the tasks with dozens of to-do list apps (I use Todois), project managing apps (I like Trello) and any kind of tool that allows you to create lists. If you don't use any, you may simple add tasks directly into the app.
2. The availability in all major platforms you can imagine. So doesn't matter if I'm on Windows or Linux PC or even can't install the app (using the web version or phone), I can just open the app and start tracking and managing my working hours.
3. When a timer is finished, be it a working timer or break timer, it has the option to change the focus of the window to the app, reminding me that I need to stop or should get back to work.
Besides that, it is so customizable (timers, sounds, shortcuts, etc.) that can fit to nearly any use-case, all up to the user! Finally, it keeps track of all your working time and breaks (which you can add notes to), so you may review them later and check out how you may improve on using your time.
PomoDone really helps me getting things done, keeps me focused and helps me improving the way I use my time. I'm relly grateful for all the team and the creator, as there is simply no other app with the all the mentioned functionalties!! Thank you!