Noyze from Tom Barrasso. Lollipop doesn't permit locking the volume rocker in the lockscreen (leading to inadvertent changes in ring volume while the phone is in a case or pocket). Noyze lets you disable the rocker in the lockscreen. It also does many more things related to device volume control but this one feature sealed the deal for me.
Tom no longer develops the app but he has kindly made it available as donationware at his website. barrasso [dot] me