The content is not identical at all web sites. Different sites may offer different search parameters and the frequency of updated information will vary from site to site. To complicate the matter further, some web sites do not have their own databases but have contracted for the use of other databases. Their value added contribution is their design of the web page. The above links, for persons in a hurry, are only suggested starting places.
Most web sites have multiple databases.
The summary table below is an attempt to show what different sites offer. The tables that follow the summary table are by category and offer additional comments. Please remember that web pages are often revised or removed without notice. The web page you are reading is intended as a starting place. It does not pretend to be an exhaustive list.