Do you need circuits for a project, or workshop use, or what? Either way, there's a ton (tonne?) of stuff on the web, guys. All you need is a little time (and, perhaps, patience), and occasionally, your “plastic” ready at hand.
Analogue, digital, PC-based, it's all there. Why not do a home-brewed using a PIC BASIC Stamp, or do it in code on your PC (in BASIC, C, whatever)? And then make an interface box (what fun!) – lots of ways, limited only by your imagination and the time available (usually the latter, in my experience).
If all you need is a bog-standard normal sinus rhythm, you can sometimes rip one out from junk equipment. If I remember rightly, some of the old S&W defibs had a built-in ECG simulator, for instance.
Anyway, the Definitive ECG Simulator Project is a hoary old chestnut that has been cropping up for at least the last 35 years. It's been in loads of magazines – and didn't guys used to build them as part of the course at Falfield? Must be hundreds of them lying around somewhere. Happy hacking!
Meanwhile, have fun with
http://www.skillstat.com/6sECG_rdm.html 
