This flash-card application allows you to use Google Spreadsheet or Microsoft Excel to create decks of cards, which are able to include sound clips or YouTube video. gFlashPro also links to a flash-card directory, enabling you to share and download premade decks with your friends or the community. There's a free version available, but the ads are intrusive enough — and the program is useful enough — that the $4.99 price is worth it. (gFlashPro, $4.99)