The DM Tool has been quite the project. When we originally started playing D&D we used the paper character sheets. But we quickly realized this wasn't going to last for long. We'd be erasing our hit points constantly during combat with damage and heals flying back and forth. In addition, we didn't want to write paragraphs of information from the player handbook onto the sheet, nor did we want to constantly be in the book to look up items, spells or features. We moved to a mock up of the paper character sheet to a spreadsheet. We then could make mouse over notes for added information. This was also time consuming. Finally Dave decided he'd make a tool that met all of our needs for a character sheet.
It took over a year for the inital functionality to come together. Then working another year or more with Josh (who used it as a programming course for high school), we rolled it out to play with our group. Since that time there have been many tweaks and additions to further hone it's usefulness. Currently they're making the Initiative Tracker portion to function smoothly during a play session. This feature also helps the DM to create encounters for groups of players.
Although the app currently only runs using Linux, I do believe that a Windows version may be coming in the next couple of years.