I'm not sure if any of you have seen this before, but this is a database of self-reported hCG levels by pregnant women. It is a listing with the DPO, average for all reporting women, lowest reported, highest reported and how many women reported their numbers. It is divided into single and multiple pregnancies.
It's helpful to know the average for your DPO so that when you are considering which HPT to use, you will be prepared that it might not show up if it is too early.
The BetaBase