THC Hydra
THC is an acronym for "The Hackers Choice"; a group of security experts based out of Germany.
Hydra is their application written for launching brute force password cracking attempts on different protocols.
Commonly used against web and ssh logins, but available for use with other types.
If you run servers and check your /var/log/auth.log text file and see a bunch of wrong connection attempts from the same location to your ssh server, they're probably using this to try and get in.
Currently this tool supports:
TELNET, FTP, HTTP, HTTPS, HTTP-PROXY, SMB, SMBNT, MS-SQL, MYSQL, REXEC,
RSH, RLOGIN, CVS, SNMP, SMTP-AUTH, SOCKS5, VNC, POP3, IMAP, NNTP, PCNFS,
ICQ, SAP/R3, LDAP2, LDAP3, Postgres, Teamspeak, Cisco auth, Cisco enable,
LDAP2, Cisco AAA (incorporated in telnet module).