Something that allows viewing message contents, copy/paste between queues and other basic explorer like functionality.
I just use the built-in MMC snap-in.
Win2k8: Start > Run > "compmgmt.msc" (Computer Management) > Features > Message Queuing
For other operating systems, you can also find it under the Computer Management MMC snap-in.
You can't do special things like copy/paste, however.
I'm starting to think maybe there isn't a good free tool...
For now I'm using Queue Explorer
This is going to be one of those questions where you just get a list of items fired back at you because 'the best' is pretty subjective. Having said that, here is the one that I use: MQMS Studio http://www.geekproject.com/tools.aspx. It is far from perfect but it allows connecting to remote queue managers and has a decent Xml view of the message payload, when it can be cast as Xml.
Googled around for the same thing myself. My purpose was only to view the message in the queue, as the mmc snap-in (Computer Management -> Services and Applications -> Message Queuing), which I guess also could count as a 'free tool', would only allow to view the start of the body (don't remember the exact limit right now). Anyways, what I wanted was a tool that could just show me the message body - preferrably as xml. And I found this one; QExplorer (http://juliusr.freeweb7.com/qxplorer.html) by Julius L. Ramos. Don't know if it's the 'best' free tool out there, but it certainly did what I needed :)
MSMQ QXplorer is on SourceForge. v.2.0 (GPL) was last updated May 2009 and works fine.