If you're looking for something more advanced, you can use the Trello API within your browser to query for information. The response is in JSON, but with a few steps you can convert that to CSV.
Example: Exporting my cards to CSV
The API route for getting your cards is documented at https://trello.com/docs/api/member/index.html#get-1-members-idmember-or-username-cards.
Log into Trello in your browser
Pro Tip: Use the JSONView Chrome extension to more easily view JSON data returned from the API.
In your browser, go to https://trello.com/1/members/me/cards
Copy everything that's returned to your clipboard (Note: If you're using JSONView, click "View Source" in the top right corner. This will be easier to copy)
Find your favorite JSON-to-CSV converter. Maybe try http://konklone.io/json/.
Paste in your JSON data and then download the resulting CSV.
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