This study compared and contrasted 64 Taiwanese college freshmen's perceptions of and attitudes toward three online vocabulary flashcard websites, Quizlet, Study Stack, and Flashcard Exchange. Four types of data were collected in two freshmen English classes in a university in Taiwan from February to April 2013. Data included online flashcard websites, classroom observations, participants' online flashcards and learning records, and interview. The study has the following two findings. First, these three online vocabulary flashcard websites mainly offered freshmen with the word knowledge in terms of form and words' meaning. Second, participants held positive attitudes toward learning and improving their vocabulary abilities via online flashcards and their related activities. Suggestions are provided to make online flashcards more effective vocabulary-learning tools: provision of user-friendly guidelines on using online flashcards, provision of flashcards with word meanings and related information, and a variety of easy exercises designed to develop learners' word knowledge.