NextBigFuture points to a neat concept for slowing a spacecraft when it enters an atmosphere. A study of the Magnetoshell concept was funded by the  NASA Innovative Advanced Concepts (NIAC). As shown in the slide below, the spacecraft would release a tethered device that creates a magnetized plasma, which in turn interacts with the atoms in the atmosphere in a manner that creates drag. The size and mass of such a system is much less than for a reentry shield and big parachutes. So it would allow for greater payload mass for missions to Mars or other bodies in the solar system with atmospheres.

Find more details in these resources:

• A Plasma Aerocapture and Entry System for Manned Missions and Planetary Deep Space Orbiters – NASA
• Magnetoshells Plasma Aerocapture for Manned Missions and Planetary Deep Space Orbiters, David Kirtley, John Slough, Anthony Pancotti – NIAC Symposium – Mar.12.2013 (pdf)