Putting message archives up for ftp access is trivial. ListProc also offers an "Interactive ListProcessor" client for telnet-like connections to the server; CREN is working on enhancing this with Mac, Windows, and X clients. LISTSERV, in its VM version, has a Gopher interface to its archives. Mailbase has both Gopher and native WWW interfaces. Various people have written hacks to interface practically every other package (including ListProc) to Gopher and WWW.
One available package that stands out for its power and ease isn't even produced by the writers of an MLM. Logika's "InfoMagnet" (for Windows and OS/2: get it at <ftp://ftp.clark.net/pub/magnet/wimag.exe>) works only with LISTSERV lists, but in that range it lets you, in a pretty Windows interface, search for interesting lists, subscribe to them, change your subscription options, and search the list archives--all without ever sending a single LISTSERV command (that is, InfoMagnet sends them for you). It even lets you set up "magazines" for individuals or organizations, for which it runs daily searches and has LISTSERV mail to you only the "interesting" messages. Nice.