I would like to find a good estimate of the sum total amount of payload that humans have put into space, that is, over all years, all space programs, and all types of payload that reached orbit. Obviously a large fraction of these quickly spent more of their mass as propellant or de-orbited, so it has no physically meaning interpretation.
The total number of orbital launches each year can be found on Wikipedia.
It would follow that if you had the payload for each one of these launches, or a good estimate of average payload, you could find the total mass that has been sent into orbit over human history. I found one source that gives the total number of orbital launches as 5,210, which I think is as of 2010. An obvious approach is to take the number of launches, and multiply by some generic payload mass to get an estimate. So far this is the best I can find online.
There is the problem of military launches, in that we categorically don't have information about those payloads. These seem to be less than 1/3rd of the launches, so it shouldn't introduce any more error than that.
I did find one source that estimates that 255 tons were put into orbit in 2007. That year is probably not representative.