From Diablo Wiki
Diii.net, formerly known as Diabloii.net, has a long tradition of making competitions of different shapes and forms. This is a summary of active and finished competitions. The dates are sorted after when the competition started.
Create a Diablo III Skill Tree
This contest required fans to create a skill tree, for a known D3 character or by inviting a new character entirely. The two winners took different approaches; one populated a Summoning skill tree for the Witch Doctor, while the other invented a new character called the Flagellant, and mapped out a number of skills for him to use.
See the Skill Tree Contest Page for the full rules, the winning entries, and more.
LOL Diablo Contestlolcats, Diii.net started a competition to add "loldiablo" comments on pictures. Diablo fans were asked to take any Diablo picture, add a caption and then upload it to the contest gallery.
Term: 4th - 14th September
Prize: 6 WWI 2008 Cards, 1 BlizzCon Ticket
Winners: Click the name for the winning entry.
Second Chance Readers' Vote Fan Art Contest
Term: 4th - 9th September
Prize: 3 Swords books by Ben Boos
Winners: 1st - johntorrio, 2nd - DefiledVisions, 3rd - akanekun
Caption This ContestWWI 2008 Tyrael WoW-pets. The four screenshots had individual articles for comments as well as winner listing and can be found here:
Term: 20th - 24th August 2008
Prize: WWI Cards
Winners: Uldyssian, arodriq, Starving_Poet, Timesink