SASL stands for "Simple Authentication and Security Layer". SASL itself is nothing more than a list of requirements for authentication mechanisms and protocols to be SASL-compatible as described in RFC 4422. IMAP, POP3 and SMTP protocols all have support for SASL.
Many people confuse SASL with one specific SASL implementation: the Cyrus SASL library. Dovecot has its own SASL implementation which may at some point be separated from Dovecot itself to "compete" against Cyrus SASL library in server side.
Dovecot SASL can already be used with:
Postfix v2.3 and later. See HowTo/PostfixAndDovecotSASL for details.
Exim v4.64 and later. See HowTo/EximAndDovecotSASL for details.
Hopefully more software will follow.