Sample chapters of textbooks focused on case marking are available for Czech and Russian. Exercises are available for both languages with feedback. The book and exercises on Polish are forthcoming.
Verbix on-line verb conjugator supports verb conjugation in numerous languages: Spanish, French, Italian, Dutch, Danish, and more. Online version of 500 Conjugated Verbs book series (unaffiliated with books).
Great online dictionary for German to English, but also German to other languages include Spanish, Finnish, Italian, French, and over 20 other languages.