There's also ProVoc for OS X which was great in its time, and still works, but has now abandoned:

http://www.arizona-software.ch/provoc/

But luckily it's been forked and now lives on as iVocabulary:

http://apps.chbeer.de/ivocabulary/

I don't know if it's strictly an SRS app or not? It's certainly smart though, and is available on OS X and iOS and has a web interface too.