Earlier I did a post about Polish Verb Conjugation and where to find resources to assist with conjugating Polish verbs. Now I'd like to talk a little about where to find resources to decline Polish nouns, and also touch on adjectives and pronouns as well, since they must be declined as well.
Wikibooks has a series that shows some tables with example nouns and their declensions. There are three different pages, one for each gender:
Polish noun cases.
Wiktionary has a more comprehensive list of Polish nouns, many of which have declension tables (you must click on the "show" link under "declension" to show the table; not all of them have this feature). Each noun is listed alphabetically but you can jump to different letters using the alphabetical links near the top of the page. There is also a brief and so far incomplete discussion of declension rules on Wiktionary at http://en.wiktionary.org/wiki/Appendix:Polish_nouns.
You can also find links to a couple of excellent grammar books in .pdf form on the Web. The first is Oscar Swan's excellent "Polish Grammar In A Nutshell." The general rules for nouns are in Chapter 3 and there are some tables of sample noun declensions on pages 24-28. The next one is "A Consise Polish Grammar" by Ronald F. Feldstein and it talks about nouns in Chapter 3. General rules for declension are on pages 41-56. Both of these books also have information on declining adjectives and pronouns as well. Of course, page numbers and/or chapters that I have mentioned here may changed if the books are revised at some point.
There is a .pdf file of a much more comprehensive grammar, also by Oscar Swan, "A Contemporary Polish Grammar." This deals with most topics in much more detail than the other two grammar books previously mentioned. Chapters 3, 4 and 5 have extensive discussions of noun declension for feminine, masculine and neuter nouns respectively and also have some fairly inclusive tables of declension. Chapters 6 and 7 talk about declining adjectives and pronouns.
There is also a good survey of noun declension on Grzegorz Jagodziński's excellent grammar site (the main link in English is here). There are links that show patterns and tables for various types of nouns, as well as adjectives and pronouns.
A brief table of the rules of declension (with no example nouns declined) can also be found (in Polish) at http://www.hamlet.edu.pl/uczen/?id=koncowki. There is another brief but somewhat confusing declension table at http://www.polish-translators.com/deklinacja.html which is in both Polish and English.
There is some talk (also in Polish) about the rules for different noun cases on the Polish Wikipedia site at http://pl.wikipedia.org/wiki/Deklinacja_(j%C4%99zykoznawstwo).
Finally, I also have made a set of flash cards called "Polish Nouns Declined" that I have developed for Anki (wow, they have recently really revamped their main page!). You have to download the software, which is free and open-source, and then go to the list of shared decks within the software program. (I also have created a deck for "Polish Verbs Conjugated" and have made contributions to Per Eriksson's "Polish-English" deck as well [though the Polish-English deck I am currently using has about 50% more material than the one on the site]. All of these shared flash card decks are completely free.)
Doubtless other resources are out there as well but this list should assist you in finding a good deal of information on declining nouns, adjectives and pronouns.