Browsers and web servers are standards and the need for instantaneous data exchange has grown. AJAX for example allows web clients to communicate "asynchronously" with remote web servers.

Comet is a Tomcat feature that goes beyond AJAX and allow real asynchronous unidirectional and bi-directional connections between client and server using the HTTPprotocol and Servlets.

Servlet 3.0 SPEC's are also providing asynchronous calls see what is possible do with them.

Tomcat-Native is a Tomcat sub-project that provides a non-blocking and very efficient SSL connections.
Tomcat-Native relies on the APR (Apache Portable Runtime) for Socket input/output and use OpenSSL to make the cryptographic layers.
Comparison of the performance of Tomcat, Tomcat + APR and httpd.