The julia-users mailing list is for discussion around the usage of julia. New users of julia can ask their questions here. As a courtesy to others, do check the archives and documentation to see if a particular question is already answered.
The julia-dev mailing list is for discussion related to the development of julia itself.
The julia-stats mailing list is a special purpose mailing list for discussions related to statistical programming with julia. Topics of interest include DataFrame support, GLM modeling, and automatic generation of MCMC code for Bayesian models.
Many Julia users and developers hang out in the #julia IRC channel on Freenode.