From NSIS Wiki
Metadl is a downloading plugin with support for both http and ftp, resuming and metalinks (including mirrors, hashes and chunk checksums). The progress display is based on code from NSISdl.
The readme describes how to use the plugin. It works almost the same as NSISdl, so it's easy to switch if you have used that plugin before. The downloading is handled by libcurl, xml parsing (for the metalinks) is handled by Expat and hashing is done by some code from XySSL. Metadl is open source and licensed under GNU LGPL.
For more information and current version see: http://hampus.vox.nu/metadl/