From the Awards website:
The Gen Con EN World RPG Awards (the “ENnies”) are an annual fan-based celebration of excellence in tabletop roleplaying gaming. The ENnies give game designers, writers and artists the recognition they deserve. It is a peoples’ choice award, and the final winners are voted upon online by the gaming public.
The ENnies were created in 2001 as an annual award ceremony, hosted by the leading D&D/d20 system fan site, EN World in partnership with Eric Noah’s Unofficial D&D 3rd Edition News. Since they were originally conceived the ENnies have expanded from an Internet-based awards selection to an annual award ceremony at Gen Con Indy. The ENnies have also branched out from their roots as an award ceremony focused upon d20 system publishers and products to celebrate the achievements of all tabletop RPGs and the publishers and products that support them.
With award categories recognizing the components that make a game great to the types of products fans have come to love, categories for fan-based websites and much more, the ENnies are the best way for fans to acknowledge outstanding effort from and to say “thank you” to the publishers, designers and artists who make this hobby great.
The Gen Con EN World RPG Awards, usually called the ENnies, are the major RPG related honor handed out at Gen Con every year. Winners are decided on by public vote, so the award reflects not just quality, but for a good part popularity, too.
The ENnies were started in 2001. To be valid for submission, products must have been released between May 1 of the previous and April 30 of the current year - e.g. May 1, 2012 and April 30, 2013 for the 2013 ENnie Awards.