If you only need to find the coordinates of a single location, use GPS Visualizer's Quick Geocoder. This utility returns a small map and a nicely formatted table of information.
If you have a large batch of addresses for which you need coordinates, GPS Visualizer's Multiple Address Locator is the solution. This form allows you to geocode an large number of addresses using the geocoding API service from Bing Maps, Google, or MapQuest. Your data can be in either a raw, jumbled, unformatted list, or in a structured table with a header row; output is plain text but can be plotted on a map or written to a GPX file.
3. Geocode simple tabular data
If your data is in tabular format (rows and columns) and contains simple data like ZIP codes, Canadian postal codes, cities, states, or airports, the best option is to format your data with an appropriate header row and then use GPS Visualizer's text/GPX conversion utility or one of the map forms directly -- GPS Visualizer has built-in databases for these types of data, so it will be much faster than having to ask Google or Bing where each point is.
Not only will the mapper or converter be faster, but you can also take advantage of features like the quantitative data form, which can colorize or resize your data points based on frequency or other parameters. (The geocoding utilities listed above in #1 and #2 will attach coordinates to your points but won't do any further processing.)
By the way, GPS Visualizer's mapper and converter will try to find the latitude and longitude of any points in your file that lack coordinates -- including street addresses. But if you do send street addresses, only a few can be processed at once, because they all must be processed by the GPS Visualizer server itself -- and it will be harder to double-check them for accuracy.
Reverse geocoding is the process of taking GPS coordinates and converting them to street addresses. At this time, GPS Visualizer is not able to do any reverse geocoding; if you need such a service, start with a Google search.