Windows 2000 Native Probably best support of windows versions. Called WebFolders
Windows XP Native

Works well once you've got a connection. When doing "Add Network Location" you should specify the port even if its default of 80. Otherwise it will sometimes create a UNC connection, which won't work.

Moves are incorrectly implemented as GET, DELETE, PUT. This is a problem if the GETable representation is not the same as your persisted representation, ie if you use templating. UPDATE 24/2/09: i just tried it again and now windows XP is issuing MOVEs correctly, at least on my machine and on milton-test. Havent gotten to the bottom of it yet, could be either because of an inadvertent enhancement in Milton (what are the chances..) or possibly a patch came through in an auto-update from MS...

Windows XP (64 bit) Native Doesnt work. Gives a message that the address is invalid.
Windows Vista Native

Webdav does not work out of the box, for at least some Vista installations. Microsoft has a KB patch to fix it. See here:

www.microsoft.com/downloads/details.aspx

Or search for KB907306

Once patched works about the same as XP. Can be a bit tricky to find what to click on to create a network location (its under My Computer, not Network Places)

Mapping a drive to the webdav share appears to work, and this might be simpler for users to setup.

One strange issue i've seen is that sometimes a user can't open a folder by double clicking it, although they can open it from the pull down menu's in Vista's address bar. Weird.

Update: Steve has confirmed that (at least for SP1) you don't need to specify the port number

Linux Gnome - Nautilus

Works well. You can enter a url into the address bar but it must specify a protocol of dav:// not http://

Also implements moves as for XP

Linux KDE I've been told this works
Mac OS Support is good and quite intuitive. However, implements some elaborate locking and committing mechanism which results in a large number of file operations and might leave temporary files on the server
Novel Netdrive/Webdrive Updated 1 sep, 2009 by Russ from ir2d. Works as of webdrive 8.22
Cadaver (linux command line client) Works
Litmus Test Results here
MS Office Found this:
http://forums.techarena.in/ms-office-support/921464.htm

"We solved this read-only problem with WebDAV in Vista SP1, IE8 and Office 2007 by installing hotfix for KB942392 (http://support.microsoft.com/KB/942392). This hotfix, probably, will be included in Vista SP2."

Looks like the hotfix is here: http://support.microsoft.com/KB/942392