I have not needed to offer myself up or find crew in a long long time. But the two that come up all the time are Crew Finder and Find a Crew. I don't know if they require a fee or not. But the structure of these organized sites are worth a small fee, as you can get feedback from others about the people you are getting ready to trust your life to.
The third is OPO
https://www.sailopo.com/
Finally, consider Facebook.
One good spot, especially if you are US-based, is the Seven Seas Cruising Association Facebook group. You do not need to be an SSCA
member to join the group. That would be a good place to post (occasionally) that you are up for
crewing. Regional FB Sailing groups are also good spots.
In our area (SE Asia), there are several groups.
There are a number of groups I am sure for the
Bahamas and the
Caribbean, also 4-5 groups at least for crossing the Pacific. Plus I know of at least 2 in the
Med.
If you are not familiar with FB groups, most require you to have some FB profile visible, so you look like a real person. Having a few shots of you on a sailboat in some capacity is useful. Some will
screen you out automatically if your FB profile is less than 30 days old. Pretty much all of them will have "joining questions" that require some answers (FB limits to 3) to also prove you are human vs a "bot" there to cause trouble. So when you request to join, be sure to scroll down and look for the questions, and answer them. There are usually rules posted for each group and most require you to state that you agree to the rules. It's a good idea to READ the rules, so you know what's allowed and not allowed.
It would NOT be a good idea to join 20 groups today and post the same message in each group. FB will normally call that spam and will put you in time-out. Other sailors will get also annoyed with seeing your same post in multiple places.
So, pick a region that you are interested in, find a sailing group, join the group, read the rules, watch the group for a week or so to get the feel of it, and make a post.
There are some significant downsides to joining just any
boat with a
skipper you don't know, so be cautious, especially if going
offshore. You should "vet" prospective boats/skippers, and they should "vet" you, if they are serious/experienced.
Good luck!