I already suggested it a few pages back, but I'll mention it again. This is a captcha that if tuned properly will be beneficial for both users and website owners - it will be easier for the users as they won't have to solve a captcha and will help the owners generate some Monero through mining: Coinhive Captcha.
Basically this captcha works in the following way:
- The user waits until his device computes a certain number of hashes. The user don't have to solve or do anything, he just should wait a little.
- Website owners mine some Monero using the computational power of the user. They should tune the required number of hashes properly so that users with slow devices (e.g. smartphones) don't have to wait too much.