The stunnel program is designed to work as an SSL encryption wrapper between remote client and local (inetd-startable) or remote server. It can be used to add SSL functionality to commonly used inetd daemons like POP2, POP3, and IMAP servers without any changes in the programs' code. It will negotiate an SSL connection using the OpenSSL or SSLeay libraries. It calls the underlying crypto libraries, so stunnel supports whatever cryptographic algorithms you compiled into your crypto package.
sessiond allows a cluster of SSL/TLS servers to share their session caches in order to prevent each node of the cluster from negotiating a separate session. SSL/TLS session is basically a set of secret values (symmetric encryption keys, MAC secrets) shared between a client and a server. The use of asymmetric cryptography required to establish new sessions is the main performance bottleneck of the SSL/TLS protocol.