Denyhosts is a small tool that frequently checks SSH daemon logs for incoming requests and put suspective IPs into /etc/hosts.deny.
For some reason, denyhosts was not running in recent days in my machine. (It should be automatically started at boot time, but was not.) Today I restarted it, and 7 IPs were immediately banned. They are from all around the world: 1 from US, 1 from Germany, 1 from Turkey, 1 from Armenia, and the rest 3 from China (respectively Xiamen University, Tianjin Netcom, and Qingdao Guangdian).
I then checked /var/log/messages, and found 5236 attacks in the last 5 days.
Yes, hackers all around the world are doing all kinds of brute-force attacks. Last summer, I found someone was using my anonymous proxy at port 3128, which had been open for just a few hours, to send hundreds of spams.