Collection of Mathematical Constants

There is the inverse symbolic calculator. Unlike the (amazingly great) OEIS which only gives exact matches, this uses an algorithm to get close to what you put in. I've not had great success with it. although I did give it $ 0.42331082512$ which is close to $\pi-e \approx 0.4233108251307480$ (so the last digit I gave it was wrong) and it figured that out.

If you go to the home page of Simon Plouffe you can see that he says that he has a more advanced version. It may be a huge Maple program which you need to download. I have not tried it out.

Steven Finch used to have a site full of essays about his favourite constants including a table, though the contents page is actually more interesting

He then turned this into a book Mathematical Constants (Encyclopedia of Mathematics and its Applications 94, CUP, 2003), formalising the essays with an expanded table on pages 543-566. The website then shrank before disappearing