You can also use Audacity to generate tones and then combine them.

The GENERATE TONE facility can be used to make tones and the QUICK MIX to combine them.

I believe that there are some materails as part of the 21C science course that use this idea for AM and FM

How to show beats on Audacity

• Generate --> Tone (440Hz, I suggest 5s is enough)
• Project --> New Audio Track
• Generate --> Tone (441Hz, I suggest 5s is enough)
• Play each on in turn - they are virtually indistinguisable

• You can zoom in (see image below) to show the difference
• Play them together, the beats jump out and slap you on the face.

Combine them to a single trace and see the beats

• Click CONTROL+A to select both tracks 
• PROJECT --> QUICK MIX
• The waveforms combineand the beats are visually obvious

Screenshots

The two tracks - can you spot the difference?

Well, ok you can if you really zoom in

The post 'Quick Mix' - Beats are obvious

Thanks to Andy Pugh