The "trunk" series represents the primary line of development rather than a stable release branch. This is sometimes also called MAIN or HEAD.
The 1.6 series aims to add more question notifications. Also, with the release of v2 of the API, inbox notifications may be used as well.
The 1.5.1 series is extremely similar to the 1.5 series but contains a fix for two rather serious bugs that somehow slipped past the testing stage.
The 1.5 series contains many internal changes including the switch to multiple threads and a web interface for configuration.
A number of significant changes are planned for the 1.4 series, including the conversion to Application Indicators.
This is the next planned stable release for StackApplet.