Best free file copier with verification?

I need to transfer a couple TB worth of files of various size between PCs (in Windows environments) and I was looking for something more then just the normal drag and drop built into Windows. Something simple that will transfer the files and also verify that each file transferred correctly via some method, maybe even a method which would involve the PC that the files are being transferred to, to do the verification so that the PC sending the files doesn't have to read back the files (I can get well over 800Mb transfer rates on my Gb LAN, but it would still take ages to write AND read at the same time).

OR, is TCP/IP secure enough with its transfers and packet verification that I shouldn't worry about it?

TCP guarantees the data over the network just fine. What it doesn't guarantee is whether your operating system and file system will get the data written to disk correctly. Use rsync or scp/sftp for that.

DeltaCopy looks half-decent, but doesn't support unicode or file paths longer than 255 characters.

You could also abuse any VCS software: Create a git repository on the root of the content you want to duplicate, commit it, and clone it on the target machine. These VCS are much better at text, though. Binary blobs are going to be somewhat slow.

If you can get a livecd with ntfs-3g on it, you could boot that, mount the Windows drive rw using ntfs-3g, and use regular rsync or scp.