I've found that good Polish language apps are pretty hard to come by, but I could have missed them.
The best Polish language book that I bought is Polski dla Obcokrajowców - it's the foundation of my (very amateurish) Polish language learning. You should check it out - learning using a book works better for me than apps, but that's a personal thing, I guess.
Writing out verb tables helps them stick in my head.
If you google 'Polish Grammar in a Nutshell', you can find a really in-depth .pdf that goes through all the basic and not-so basic facets of Polish grammar. It's pretty good for when you want to melt your brain a little.
If you are on Android i found a really fun way of learning Polish words.
The app is called Funeasylearn Polish. Look it up! its really good in teaching you how to say words, read them and spell them in Polski!