From NSIS Wiki
Source code can be obtained using SVN or using the packaged tarball available in the file list.
Prior to version 2.07, source code was included in the installer.
Using the development files you can try the latest cutting-edge features and fixes. These files have not been tested so they should not be used in production environments.
See the development change log for the changes since the last release.
Change log generated Fri, 20 Apr 2012, 19:16 GMT
To get the changes right to your mailbox, subscribe to nsis-commits.
You can use SVN to get the latest development source code. You will always get the latest files and only updated files will be downloaded.
SVN is also easier for debugging, because you can download and compare older revisions and merge your own changes.
Details about SVN and access to the NSIS source code can be found here.
You can also browse the SVN repository using a web interface.
The nightly build ZIP archive is compiled from the latest development files. It's updated daily at midnight UTC.
The latest build was generated on Wed, 26 Jan 2011, 00:37 GMT