I have a lot of untracked files. I am pretty sure that most of them I can delete, however... you know, backup could be helpful ;)
What you are doing in similar situation?
The following command will create a tar archive in your home directory of all of the untracked (and not ignored) files in your directory:
git ls-files --others --exclude-standard -z | xargs -0 tar rvf ~/backup-untracked.tar
If you're going to use this technique, check carefully that git ls-files --others --exclude-standard on its own produces the list of files you expect!
A few notes on this solution might be in order:
I've used -z to get git ls-files to output the list of files with NUL (a zero byte) as the separator between files, and the -0 parameter to xargs tells it to consider NUL to be the separator between the parameters it reads from standard input. This is a standard trick to deal with the possibility that a filename might contain a newline, since the only two bytes that aren't allowed in filenames on Linux are NUL and /.
If you have a huge number of untracked files then xargs will run the tar command more than once, so it's important that I've told tar to append files (r) rather than create a new archive (c), otherwise the later invocations of tar will overwrite the archive created just before.