Another vote here for Darik's DBAN
That should be good enough for most uses.
Off on a slight tangent though, its worth remembering that if someone really wants the data on that hard drive, physical destruction isn't enough. Even if you drill holes in the platter, or hammer it to splinters its usually possible to reassemble a working drive image if you have the right kit. Theres enough data redundancy to often overcome the "missing" parts.
And by "the right kit" I mean one of these
Scanning tunneling microscope - Wikipedia, the free encyclopedia
There are companies in the UK that can do it, though its not cheap....
They scan all the fragments with it and then use a rather powerful PC to electronically reassemble all the parts. By locating the charge on individual atoms on the platter, they can make a good judgement of the data