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.

I tried Quizlet for many months, but actually just recently switched to Anki. The Quizlet UI looks better on the surface, but Anki's UI is faster and more... utilitarian.

Who cares? The point of Anki is the SRS, the rest is mere sparkles. Sparkles are awesome but Anki is a full featured, usable, extensible SRS.

If you still want SRS it's pretty easy to export from Quizlet to Anki:

Find the export button on the upper right of Quizlet's website, and select tab delimited and new line termination. Copy paste this into a text editor. Now you can just click the import button in Anki and it's pretty straightforward!

One can also try http://studyblue.com/

Personal ranking: Studyblue > Quizlet > Anki

I use a mobile app for Anki, and I have audio for some of my flashcards.