Idea and Solution Repositories
If you are looking for open datasets for your study, assignment, presentation, or application design and development these are some of the resources you can use.
You can also use some of these repositories to archive your data (see Repositories for Research Data)
General Data Search Engines and Registries
DataCite - helps locate, identify, and cite research data
General Repositories for Research Data
Discipline Specific Repositories for Research Data
List of Discipline Specific Repositories from Nature
UCI Machine Learning Data Repository - maintains hundreds of data sets as a service to the machine learning community.