Remote control central does a great job in this area.
http://www.remotecentral.com/
They even include the codes for many things the companies themselves deny having discreet codes for (ie Replay TV's)
Good luck!
As an aside, you can get away without discreet codes if you can find a key that gets the unit into a 'known state', ie on or off, always. VCRs are famous for 'play' always turning them on. In this case, you can use a macro to hit play (to make sure it's on) then stop (if you want it just on) or then a pause and the power button to make sure it's off. It makes the remote work slightly slower, but not anything you should be bothered by.
My pre/pro (Linn 5103) doesn't have either so I've got it set up in a generic start screen that welcomes you to my HT, once you hit this, it triggers the standby (turns it on). I then only have that code again in the macro for system off that takes you back to the startup screen (and in a special power button screen just in case..). It's not perfect, but in two years I've only had to use the power button for it like twice, so I'm pretty happy with that.
