Open Broadcaster Software is available in both 32-bit and 64-bit versions. As well, the source code is available for all to use and change under the GNU General Public License v2.
If you like, you can also check out this project on GitHub (NOTE: soon to be deprecated. Instead, check out the new multi-platform version we're working on, with better code, better features, modular design, and better API). Older versions can be accessed on the GitHub releases page.