This page will let you convert a large number of locations in various coordinate systems (such as latitude/longitude) to other coordinate systems. Paste them into the input box (latitude/longitude should be separated by either tabs or commas followed by new lines) and hit the 'Convert' button. The box below should contain the converted coordinates. Copy the output into a CSV file for easier manipulation.
If you have a Google Maps API key, you can also reverse geocode locations to addresses