NetBeans source code has been released to the open source community under a variant of the Mozilla Public License (MPL). The license, called the Sun Public License (SPL), is functionally equivalent to the MPL with only trivial changes such as mentioning Sun rather than Netscape. You can view the exact differences between the Mozilla Public License and the Sun Public License in two forms -- the hackers diff and the lawyer's diff. The full text of the SPL can be found at http://www.netbeans.org/about/legal/spl.html. The NetBeans source code is distributed under the Sun Public License (SPL) to encourage the use and distribution of the NetBeans code to all developers. The SPL aims to balance the interests of the open source development community as well as commercial programmers. Some licenses, such as the BSD, do not require that modifications be given back to the Community. The SPL requires modifications to be given back so that the entire community benefits. Other licenses discourage commercial usage of the code. The SPL balances the needs of all developers in a way that is appropriate for the NetBeans community.
Source: NetBeans User FAQ