I am still at the early stage of learning the Polish language, so therefore cannot confirm the content accuracy contained on these websites. However I am finding them (and others), very helpful in my quest to learn this interesting language!
The first website contains a good explanation of the 7 Polish cases. Plus lots more useful information and instructions on Polish Grammar. Including handy PDF's, 'Summaries of the endings of nouns and adjectives in all seven Polish cases' and 'Present tense Verbs': http://speak-polish.net
A briefer explanation on Polish grammar the can be found: http://polishgrammar.com/noun-case-identification-ll/
Practice Polish grammar quizzes and on-line tests: http://www.learnpolishfeelgood.com/polish-nouns-pronouns.html
These 3 videos run through the declensions, both in single and plural:
Another very useful & informative Polish website is: http://www.polish-dictionary.com
This website has so many free Polish TV programs: http://serialnet.pl
For up to date News video clips in Polish: http://www.tvn24.pl/wideo
Polish TV Channels On-line: http://www.tv-tube.org/tvchannels/country/poland
Interesting and in parts funny video on the Polish Language Ą-Ę case study: https://www.youtube.com/watch?v=AYWTnbnEWlU
I hope these are of help to you.
Lastly, I would be grateful for any advice you can offer me.