That depends on what you mean.
If you are looking for a site that gives declension of all nouns - you can use Grammatical dictionary of Polish on-line (you can switch its interface to English). This is a great dictionary: it lists ALL the forms of a HUGE number of words (over 334.000 - more than any other dictionary of Polish), some of them even purely virtual (I mean words that are almost never used). And If a word can be used in various functions (eg. may be a person's family name) - it is listed separately for every function, as it has usually different flection in every function.
However, if you are looking for information which case should be used after which verb, then it is a harder problem. I do not know of any extensive list of verbs on-line. I have a few proposals.
- You may use Słownik języka polskiego pod red. W. Doroszewskiego on-line - these are scans of an old, but very extensive dictionary, that lists about 125.000 Polish words with many samples of phrases for each of them. It does not say what case should be used after a verb, but it gives many samples of correct phrases for each, so this can be deducted out of these samples.
- Easy Polish Language Shool lists some basic verbs with the cases they require: http://easypolish.com.pl/images/dla-obcokrajowcow-lekcja2/czasowniki-i-przypadki.pdf
- Some verbs with the cases they require are listed here: http://www.langsites.com/Verbs.htm
- I listed a long description of how to use Genitive in Polish (probably the most complicated case), with a list of common verbs that require Genitive. And here is an article about all the cases with some sample verbs.
- You may use paid dictionary Uniwersalny Słownik Języka Polskiego (there are options: https://usjp.pwn.pl/krok1.html - I use it from time to time) or buy Wielki słownik języka polskiego PWN on Pen-Drive, with USJP as a part of this. USJP gives full information about each word, including cases that are required by given verb.
- The case after verb is also governed by the preposition used. Here is a great manual of Polish Language that lists prepositions by case they require. And here is a great article on that, written by one of the Contributors of the Polish Course.
(If I find other options, I'll add information here)