Those eBay 11 cent connectors that GRJ found are quite the price!
For DIY electronic tinkering, I've found a 9V battery makes a convenient power source for simple measurement or signal generator circuits. I've found the wireless style of 9V connectors can be handy when building hand-held "probe" widgits that uses the heft of the battery...rather than a dangling circuit But these wireless (aka PCB-mount) connectors are harder to find and over $1 at DigiKey - rarely find them cheap on eBay. Hence I find it worth the effort to recycle them from a spent 9V battery. Here are some 9V test circuits scattered about the bench showing different styles of 9V connectors.