It's pretty easy (and free!) to download R and install it on your own PC, Mac or Linux machine, but if you don't have one of those or simply aren't ready to commit to installing it, you can now try it out online. R-Fiddle (from DataMind) provides an easy-to-use interactive R console that you can run from your browser. Here's an example:
The top pane is a script editor; you can run the script by clicking the “Run Code”. You can also run a single line of code by pressing Cmd-Enter in the script, or by typing directly at the prompt in the bottom page. If you ask for a help page (as with ?fft above), you'll be directed to an interactive R help page where you can even run the examples interactively. And if you click the “Save” button, you can even share your script with others via an embed code, as I've done below:
It's easy to get started — just visit www-r-fiddle.org.