Look to the media: what are people talking about right now? What solutions could we present for their problems? How can we take what they like and make it better?
Look to consumers: what are people unsatisfied with? What niches do consumers need more education or resources in?
Look for the gaps: what technologies can we take and combine to make them more effective, or of more value? What features can we add?
Look for trends: what is popular right now?
Look for niches: are there any topics/areas of interest that have been relatively untouched? Is there a product or service that works well for a specific niche that we could reconceptualization to suit another niche?
Look at previous hackathon ideas: do they open up other issues that could be addressed?
Get creative! Talk to people, ask questions!