libc, but I didn't find any command line utility to do this.
Though I've written a piece of C code, I'm still wonder if it already existed somewhere?
There is a version of usleep as part of the nosql RDBM package. The installation of nosql requires procmail and postfix, so it is a bit heavy handed to install all that for one tiny utility.
However, if you locate and download the .deb package without installing it, you can use tar to unwind it (a deb package is simply a compressed tar file containing compressed tar files). Extract only