Android library projects: just do it.

Library projects in your Android application reduce the amount of code you have to write. This can only be a good thing - we know that all software has bugs, therefore the more you write the more bugs there are.

In Eclipse:

• Create an Android project, right click and click properties.
• Click Android
• Check the "library" checkbox.
• Similarly open the Android properties for another project.
• In the libraries section click Add and select the project that you made a library.
• Repeat for several other projects and your library project will be used by all - reuse is a Good Thing (tm).

More info here.