You can uncross a crossed cheque (that you've written yourself) by writing 'Crossing cancelled' over it and adding your signature, enabling it to be paid into the bearer's bank account. However money laundering regulations mean that all bank transactions must now be traceable, so you can no longer write "Pay cash" (other, of course, than when cashing a cheque yourself).
The 'traceability rule' is so strict that when, for example, I took about £120 worth of old pound coins into my bank (while they were still valid in shops, etc) to swap them for the new £1 coins, I wasn't allowed to simple hand over my old coins and receive new ones in exchange. I had to pay the old coins into my account and then make a withdrawal in order to receive new ones.