SHOUTcast DNAS and IceCast are both streaming server applications. Unless you're doing anything really exotic, SHOUTcast DNAS and IceCast essentially serve the same purpose, and they're both free to use.
IceCast offers a few more advanced features and is open-source software, but it's usually harder to install unless your Linux distribution offers a pre- built IceCast package.
SHOUTcast DNAS is much easier to install (as it's self-contained in a single binary file), but it's proprietary and closed-source.
Both products are typically very stable and reliable, offer similar core feature sets, and are widely used by many high-traffic streaming radio stations.