The latest Slic3r is now out (Version 1.0.0RC1) - it's a release candidate, but incorporates a large number of exciting feature and performance improvements.
Some of the key improvements include:
- Automatic STL repair
- New ways of handling support material
- Filament volume calculation
- Improved speed, thanks in part to having significant portions rewritten in C++
- Reduced memory usage
- Better infill and bridge paths
- Improved Makerbot compatibility
- Ability to pass custom G-code as command-line options
Check out the full release note.