As the thread title requests - If you have some free CPU power then try mining Gapcoin !
Gapcoin is a prime number based p2p cryptocurrency and the largest, decentralized prime gap search.
The original developer has sadly passed away, however
the project is ongoing at: https://gapcoin.club With increased hashing power (and continued development) the Gapcoin network
will discover new Prime Gaps.
See:
https://gapcoin.club/primegaps.phpLets try to get cryptocurrency back in the news for all the right reasons again. Do it for the math and not just for the money.

Gapcoin thread here at bitcointalk.org - see:
https://bitcointalk.org/index.php?topic=822498Thank you.
EDIT: 5th June 2020 updated links and info.