Redmine is popular open source project management and issue tracking software. It has a number of great features, including Gantt charts, project wikis and repository browser. There are some negatives too. For example, there are no client management and invoicing tools inside Redmine (they are only available as third party plugins, such as RedmineCRM). Also, there are at least six different Android apps released by different publishers, related to Redmine, which is confusing. If you are looking for a free Redmine alternative, consider Bitrix24.
* Please note that the information may have changed since the publication. For current prices and features please visit Bitrix24 pricing page.