Contact
One address, read by the person who builds the site. Bug reports, wrong results, tool requests and data requests all go to the same place.
What to write about
A tool giving a wrong answer is the most useful thing you can report — include the input, what you got and what you expected. A tool you need that is not here is the second most useful; the catalogue grows from exactly those requests.
Broken pages, typos in either language and unclear explanations are also welcome. There is no ticket system; it is an inbox.
Data requests
To see what is held about your account or to have it deleted, write from the address you registered with. Deletion removes the account, favourites and usage records; see the privacy policy for what is stored in the first place.
Replies usually take a day or two. If you are reporting a security issue, please say so in the subject line.