b2utiful night

Even though I haven't tried all of them, here are some of the best free ones out there.
So if you have a smartphone, make sure to check them out.

(All links are for Android..)

VOCAB, FLASHCARDS, etc
https://play.google.com/store/apps/details?id=com.htcheng.enkremember
https://play.google.com/store/apps/details?id=com.startline.abc123
https://play.google.com/store/apps/details?id=com.bravolol.bravolang.englishkoreanflashcards

DICTIONARIES
https://play.google.com/store/apps/details?id=en.ko.bestdict
https://play.google.com/store/apps/details?id=com.nhn.android.naverdic (ONLINE only)
https://play.google.com/store/apps/details?id=com.omega.dic.en

Basic GRAMMAR
https://play.google.com/store/apps/details?id=edu.ckcc.koreanbasicgrammar90

TOPIK
https://play.google.com/store/apps/details?id=com.ICanDo.TopikKorean
https://play.google.com/store/apps/details?id=br.com.hd1.graziani (TOTALLY WORTH BUYING! at least for me~)
https://play.google.com/store/apps/details?id=com.hongik.TOPIK_5256