Different generations of the BSD and Apache licenses have had attribution clauses to various levels of strictness. (Older ones were more strict). Neither are copyleft (Like GPL/CDDL) forcing future improvements to be open sourced. CDDL may be interesting as a way to preventing improvements to a code base from going "dark" but still being non-copyleft compatible. (Hello Sun/Oracle). Another commenter mentioned MDL which CDDL was based on for Sun's needs, that is also worth looking at.