Here's a brief overview of four of the most popular pieces of malware:
More details at ZD Net.njRat – a Windows remote-access trojan that first emerged in 2012. Its source code is available online and, despite its age, it remains a popular form of malware, especially for those targeting older systems. SpyNote – a freely-available Android-based RAT containing keylogging and GPS functionality, which first emerged in 2016. GandCrab – a prolific form of ransomware which offered an affiliate scheme that allowed users to easily distribute file-locking malware. The GandCrab authors announced their retirement in June 2019, claiming affiliates had made billions of dollars. It's the only ransomware strain that was highly popular with dark web forum users.
DroidJack – an Android trojan from 2014 which sold lifetime licenses for just over $200. However, cracked versions of it are far cheaper on underground forums.