This is by no means a comprehensive list, but could serve as a starting point in your searches.

• Datahub.io central data portal of Open Knowledge
• Datacatalogs.org a comprehensive list of open data portals
• Links to Open Data sites in the U.S. and international
• WikiData.org from Wikimedia projects
• PublicData.eu European government datasets
• Planet Data.eu

• Open Data Ireland
• Open Gov't Data UK
• French data-gov.fr citoyen
• U.S. Open Data portal
• Canadian Open Data portal

• NYC.gov/data - New York City
• dataSF.org - San Francisco
• data.wien.gv.at - Vienna
• daten.berlin.de - Berlin, Germany (apps )
• data.london.gov.uk - London, UK (apps )
• opendata.paris.fr - Paris, France (apps )
• toronto.ca/open - Toronto, Canada (apps )
• montrealouvert.net - Montréal, Canada (apps )

• GitHub and Government
• InfoChimps
• Data Marketplace
• Factual

• awesome public datasets - @caesar0301's list on GitHub