Get ethereal , from http://ethereal.zing.org/ -- it's open source, and includes links under downloads for binary distributions for Win32 and a variety of Unix distribs.
...although its SMB dissection isn't yet as complete as that of, say, Microsoft Network Monitor. Fortunately...
The Win32 distribution includes "editcap.exe" to convert from libpcap/tcpdump formats to a variety of commercial analyzers.
...one of those formats is Network Monitor format (which the library that Ethereal, Tethereal, and editcap use can also read).
editcap source comes with the Ethereal source distribution, and is probably in at least some of the UNIX binary distributions.
Note, though, that not all of the UNIX binary distributions are the latest version of Ethereal; for example, the one at sunfreeware.com for Solaris 8/SPARC is just 0.8.4, but Ethereal is up to 0.8.12 at this point.
pktt files can, of course, also be read by anything *else* that can read tcpdump files.