There are a number of freeware audio generators. Here's one from "Tucows" which is a trusted source. (I like this one since it appears from the screen shot to have a selection for different wave forms.)
Test Tone Generator 4.2
http://www.tucows.com/preview/240287
As the above member stated, any suitably powered stereo amp should be satisfactory for driving a conventional speaker.
BTW, I remember the sweep generator I wrote code for that runs in my Commodore 64. I used to use it to test my stereo system (also check tune of musical instruments, etc.). Worked really good too. Could put out a steady wave form of selected frequency, could sweep a range of frequencies, could select the sweep speed as well as select different wave form types, adjust volume output, etc. all in real time on the fly with on screen display too.
Lots of fun writing the code.