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You can participate in those giveaway events in three basic ways: 1/ as an organizer, 2/ as a product supplier and 3/ as a taker. You should join every event that attracts your attention, especially if the gifts are in the niche or medium you're very interested (e.g., blogging, podcasting, article marketing, video marketing, social marketing, etc.). It won't cost you anything, except your time. Yes, that's the catch (you knew there will be one...) - in order to get any gift you will have to sign up for the mailing list.