TimeZoneDB provides free time zone database for cities of the world. The database is licensed under Creative Commons Attribution 3.0 License. It contains countries name, time zones, abbreviation, GMT offset, and Daylight Saving Time (DST). The data is available in CSV and SQL format. You can download and implement into your projects for free.
API access also available to query time zone information. You can get time zone data by latitude & longitude, or zone name. You need to register a API key to access the API.
An on-line web application that shows you what time it is in every other time zone around the world relative to the current time. Handy if you're trying to figure out if you should ping someone at a particular hour.