As you may know, Task Coach is a free flexible open source todo manager featuring hierarchical tasks. It grew out of frustration about other programs not handling composite tasks well. In addition to flexible composite tasks, Task Coach has grown to include prerequisites, prioritizing, effort tracking, category tags, budgets, notes, and many other features. However, users are not forced to use all these features; Task Coach can be as simple or complex as you need it to be.
Task Coach has reached 1.3.32 with lots of bug fixes. It has been added to Ubuntu 13.10 Saucy’s universe repository, so it can be easily installed via Ubuntu Software Center.
For Ubuntu 13.04 Raring and earlier version, deb package can be download at sourceforge.net