Czech dictionaries and Wikipedia
There are a couple of convenient online English-Czech dictionaries. I used slovnik.cz for a long time, but it’s just a simple wordlist, without noun gender, sample sentences, &c. WordReference.com/encz/ has much more information (it’s missing the genitive form of nouns).
The Internetová jazyková příručka, or the older (1960s era?) Slovník spisovného jazyka českého, put out by The Institute of the Czech Language of the Academy of Sciences of the Czech Republic, is a much richer dictionary (all in Czech).
One of the best resources for language learners is the multilingual Wikipedia. If you look at the left-hand navigation menu, there is a Languages menu that frequently offers a Čeština version of the current page. This is a really useful way of gaining perspective of what a word or idea means in a Czech context. For example, we can click over to see that The Lord of the Rings is Pán prstenů in Czech. Google Chrome offers automatic machine translation of Web pages, which is another way to make more sense of Czech-language Wikipedia. Search for a term in Google.cz and see what comes up, then click on Obrásky “Images” to see some pictures. Google Translate is machine translation, and thus riddled with inaccuracies and weird errors, but can be useful for some limited purposes.