https://www.genuki.org.uk/
Probably the best source of information for the UK and Ireland. If you want to go straight to the country/county, then click on 'United Kingdom and Ireland' in the top left corner.
Be aware that this site can be very addictive and lead you to all sorts of places far removed from your original query.

https://www.familysearch.org/
An excellent source of baptisms and marriages from approximately 1600 to 1850. Be aware that 'submitted' entries can be unreliable, and that 'about' can mean up to 20 years either way!
Also one way through which you can view parish registers and other documents worldwide. (See 'find a Family History Center near your home'.)

https://www.freebmd.org.uk/
First port of call for tracking down post 1837 birth, marriage and death certificates, which can then be ordered on-line from https://www.gro.gov.uk/gro/content/certificates/

dixons.clara.co.uk/Certificates/indexbd.htm
The site which tells you almost everything you could ever want to know about BMD certificates.

https://freereg.rootsweb.com/
An ever-increasing site listing transcriptions of parish registers (PRs) in England, Scotland and Wales. Transcribers always needed.

https://freecen.rootsweb.com/
Another ever-increasing site, this time listing census entries for England Scotland and Wales.

cyndislist.com/
One which is often not mentioned nowadays, but which is a mine of world-wide information. It's not quite true to say that if it's not on cyndislist it doesn't exist, but almost.

And should you ever find a site which leads to a 'page not found' try accessing the site via The Wayback Machine at
https://www.archive.org/index.php
There's also lots of stuff to be found by clicking on the 'moving images', 'texts', etc links at the top of the page.
Pam