We're huge supporters of open source at Vector, and have contributed lots of code back to the open source community. In addition to patches and pull requests on various projects, we've released the following popular projects of our own:
VMG Chosen Member is a fieldtype for allowing the selection of one or more members inside an entry. It's specifically designed for use on sites with a large amount of members, in cases where a regular <select> dropdown with thousands of <option>s would decrease publish page performance.
An extremely flexible plugin for ExpressionEngine that allows for more flexibility when nesting any module and plugin tags within others, including the ability to add variable prefixes to any tags. VMG Nested supports third party modules and plugins.