Forks
In 2011, one of the most active Redmine developers, Eric Davis, with a group of other contributors from a company named Finnlabs, forked Redmine into ChiliProject. Unfortunately, this project was recently discontinued.
However, while working on ChiliProject, Finnlabs decided to make a custom version of this application. Eventually, having understood that ChiliProject was not good enough for them, they made their fork public and named it OpenProject. Right now, this fork is actively developed and already has quite a large community. Finnlabs also changed the style of the project—they made it more commercially oriented and created the OpenProject Foundation, which is funding it. These changes had a positive effect on the fork, which now looks quite cool and promising. However, it's obvious that the project is too young to replace Redmine. Also, it has not proven its durability yet (ChiliProject looked promising too).
Note
Check out OpenProject at https://www.openproject.org/.