Nexus Mod Manager (NMM) is free, open source and it's based under the GNU General Public License.

It's latest version is still in open beta so, despite some bugs that might rarely appear, it's a consistent, reliable and easy program to use. Although you can install mods manually, it's much more practical to use a manager like this one.

For developers willing to help with improvements for NMM, you can visit the developer's page at SourceForge.