Diarly is a wonderful personal journal app built by PureForm Studio. I use it on a daily basis because of journaling's proven benefits for personal improvement.
After suggesting a few typographical improvements, the team contacted me to re-design the UI of the desktop and mobile app. Needless to say, I was thrilled, and a great project started.
This was such a great project to work on, especially as I could see the changes improvements on an app I already used every day. This also allowed me to take both the position of the designer and the user, an experience from which I learned a lot, and which greatly improved my design skills and processes.