LISTSERV and CREN ListProc are commercial products, licensed by the year
(more or less -- each has several licensing plans and you should contact
their vendors for details).  In exchange for your money you get a product
that's stable, documented, and pretty simple to set up, with paid support
people to help you out if you get into trouble.  The other MLM's are free,
with varying levels of support from their users and maintainers.  Each
package has a mailing list for users, on which you can ask questions like
"digesting doesn't work, what do I do?"  Many people are satisfied with this
sort of support; you may or may not be.  Talk to current users of each
package you're considering to get a feel for whether you're comfortable with
its combination of administrative burden and software cost.

Some parts © 2009 Advameg, Inc.