I got a fairly significant patch to DrProject today. I wanted to show it to the other developers so they could give it a look over. How'd I do it?
$ patch -p0 < patch
... check that it works ...
# Create a new branch for the patch
$ svk cp //drp/trunk //drp/branches/patch -m "Created branch for patch"
# Do an in-place switch of my repository (which takes about 2 seconds,
# versus the minute or two it would take to check the entire tree out)
$ svk switch //drp/branches/patch
# Because switch doesn't revert modified files, the changes are still here,
# ready to be checked in
$ svk ci -m "Checking in patched code"
Now they can use DrProject's fancy online changeset viewer, checkout the code for themselves, use svn diff... The list is endless.
Now that rocks SO much more than mailing around .diff files (and makes life happier when it comes time to pull it back into trunk).