The best option to wipe the content from a drive is to destroy it by use of force. Either melt it in fireplace or drill it in many places. Problem is, nobody is going to buy the disk after that.
Software-wise, there are quite a few tools both free and paid that would overwrite the data using either zeros or random sequence for you. Once overwritten, the information is lost forever.
To get the same effect yourself as with the secure erase software, just format the hard drive and then fill it to its full capacity with any insecure data (similar to many copies of Mickey Mouse DVD iso). When the big file fits no more, continue adding some smaller files.
On Full Encryption drives, just changing a password achieves pretty much the same effect as after a secure erase.