Small things built while working on something else, then cleaned up because they turned out to be useful. Free, no accounts, no tracking on the inputs. Where a tool quotes a number, that number was measured rather than estimated, and the method is written at the bottom of the page so you can check it.
Runs in your browser
Paste an X.509 certificate and see its structure: every object with its byte offset, object identifiers decoded from raw bytes, key usage bits translated, and a mode that diffs a broken certificate against one that works. Nothing is uploaded, because the parser is in the page.
Built on measured data
Work out whether a post-quantum certificate chain fits inside TCP's initial congestion window, and which client stacks can actually complete the handshake. The usual "keep it under 10KB" advice states the limit on the wrong quantity.