SPIZD stands for Stress Probing Invasive Zap Destructor; it's a command-line stress test tool used to determine how many simultaneous (concurrent) connections servers can handle. Protocols: http, pop3, pop3s, imap, imaps, smtp, smtps, ssh, radius.
WebLOQ IO is a Java NIO framework which allows for tens of thousands of concurrent TCP/IP connections through a very small thread pool (assuming proper OS and network settings). There is built in support for SSL and non-SSL connections.