Not only a world clock, it will also convert times from one timezone to another.

WorldTimeAPI is a simple web service which returns the local-time for a given timezone as either JSON or plain-text. Some additional information is provided, such as whether that timezone is currently in Daylight Savings Time, when DST starts and ends, the UTC offset, etc. Does not require an account or API key. Free.

REST Services that will return current date/time in JSON for any registered time zone. Seems pretty straightforward and easy to use.

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.

python module that makes handling dates and times significantly easier. Get, parse, and convert datetimes in various forms, such as time_t, natural language ("23 hours from now," "last year"), and various RFC formats. Can also do timezone and arbitrary time interval algebra. Cross-platform.