I do not know of any but that doesn't mean that they don't exist. I'm not one to go totally negative on the idea of DIY, if nothing else it can be well worth the learning exercise.
As I can't afford , $100s to buy a bench meter I was wondering if there are any open source projects around ( and no not the CERN one thanks !!) I'm looking for one with good accuracy and resolution ( 5-6 digits )
I don't really think that will happen. You end up having to achieve a very high level of performance and not having the hardware and design experience to get it done.
A better approach might be to build a meter in the 4 digit range out of a chip set specifically designed for multimeters. You should be able to source the components at reasonable prices and not be stretching your developing design skills. A meter with 4 to 4.5 digits should be plenty on many benches, remember the world started out with analog meters.
dave
I actually like the idea of more open source instruments. If you are going this route I'd certainly consider a meter design that is easy to achieve. I know many people are wowed by the latest high resolution meter with its great precision, however I'm going to upset a few people and say for most users the high resolution isn't needed. Obviously the end use dictates what you actually will need.
The stark reality is that with a low end budget you will get a low end meter. As such it is better to compromise on digits and spend more effort on software and getting as much functionality out of your chip set as is possible. Also a focus on MMI (man machine interface) and a really decent display might make sense.