Slavic and Baltic Heraldry

Russian Heraldry

Heraldry was not adopted in Russia until after 1600. However, some information about tribal symbols and identification marks in Russia can be found in the following article:

Period Russian Heraldry, by Paul Wickenden of Thanet.
Polish Heraldry

Herby w legendzie obrazowej o. św. Jadwidze, 1353 r.
This page contains thumbnails of heraldic images from a text about Saint Jadwiga dating from 1353. The site is all in Polish, but the images are straightforward

Polish arms in the Armorial Bellenville.
The Armorial Bellenville is a French roll of arms from around 1380.

Polish arms in the Armorial Gelre.
The Armorial Gelre was compiled between c.1370 and 1386 by Geldre Claes Heinen. It include over 1750 achievements of arms. This page has extracted the arms of Polish knights.

Herby rycerstwa polskiego.
This is an 1858 reprint of a book originally published in 1584. It is hosted by Googlebooks, and so the link may not work for people outside of the US and parts of Europe.

Polish Web Armorial, by Kaj Malachowski.
Only some of the entries are dated, so this source should be used with care.

Polish Nobility and Its Heraldry: An Introduction, by Piotr Pawel Bajer.
The two sections on Polish medieval arms are the most useful. The author doesn't list the arms, but he does list sources which contain Polish arms, with brief descriptions. This is a great starting point for research.