Two purchases come to mind.
Firstly, without a doubt the i5 2500k! had my moneys worth and more over the years, bought it summer 2011, been in multiple builds over that time and performed flawlessly at 4.4Ghz on stock volts, and although not the fastest, its still arguably relevant to this day. Had my Z68 motherboard not started to fail (all but 2 usb ports are shot) I wouldnt have even bothered upgrading, however sad as it is, the time has come to retire it.
Secondly, SSD not any one in particular, just in general, such a huge performance boost over traditional HDD.