Is there one? Since the Polish monarchy in 1795 was elective, how is it determined who might be a pretender to the throne?
As a side question, are there still any Piasts (the pre-commonwealth dynastic rulers of Poland) kicking around?
There's no pretenders because the throne was not inherited.
However, the 3rd May constitution of 1791 reintroduced hereditary monarchy, transferring it back to the Wettin dynasty, who were elected independently as Augustus II the Strong and Augustus III. Augustus' heirs could argue to have a degree of legitimacy.
Frederick Augustus, though he initially refused being King of Poland, was installed by Napoleon as a Duke of Warsaw after the partitions. He renounced the title at Congress of Vienna, and the Russian tzar became King of Poland.
Initially, the Russians did enjoy a degree of acceptance, but eventually the Polish autonomy was abolished, thus ending Kingdom of Poland as any separate construction (Lithuania has been already absorbed into Russia).
As for the Piasts, there are probably some German aristocrats who trace their lineage to the Silesian Piast branch, but there are no direct Piast descendants.
Source: God's Playground by Norman Davies, Liberty's Folly by Łukawski.