It was nice to find out in Jeremy Uejio’s Blog that 0xDEADBEEF is used on Linux (I assume it happens with debug versions) when you free memory.
According to this source, originally DEADBEEF was used in order to fill newly allocated memory.
On my Windows machine neither fresh nor deallocated memory is fancy.
Only 0xAB, 0xFE and nothing really meaningful.
For more historical stuff see The Jargon File.