One of the best things Google has done for developers over the last year was the Google APIs explorer. It allowed developers to try out the various APIs that Google offers while giving them tips on how to best take advantage of it. It didn't seem like it could get much better than that, but Google has a habit of proving me wrong.
Google has introduced a brand new look for the APIs explorer. Their claim is that its "easier and more fun to navigate." I can attest that it's much easier to navigate, but fun is purely subjective. You'll have to try it out for yourself. The facelift is not the only new thing hitting the APIs explorer as it is also getting some major new features. It includes an indexed history of your API calls, an improved editor for the request body, and a search box.
Those are all nice, but what about new APIs? The old APIs Explorer just had support for a little over half dozen APIs. There's been a lot of work done on APIs over the last year so surely there's a few more, right? Try a lot more as the APIs Explorer now supports 26 APIs. Google hopes to keep adding more APIs regularly.
Here's the full list of APIs that you can play around and experiment with right now:
If you're new to a specific API or just the Google APIs in general, I highly recommend you check out the APIs explorer. It's a great tool for getting acquainted with the APIs that you'll probably be using in various applications around the Web. As always, check out the documentation for all the info you need to get started.