Well, traditionally a ballad was simply a song that told a story. But modern ballads of course are those sappy things bands play to get chicks.
Usually period a slower instrumentation that's usually cleanly played (either clean guitar or piano) with some sort of "heavy" part in the middle.
Some favorites:
Edguy - Land of the Miracle, Save Me
Queensryche - Silent Lucid, I Will Remember, Anybody Listening
Skid Row - I Remember You, Quicksand Jesus (best song they ever wrote), In a Darkened Room, Wasted Time
Metallica - Fade to Black, Nothing Else Matters
Megadeth - In My Darkest Hour
Motley - Home Sweet Home
Priest - Diamonds and Rust (Acoustic version), Dreamer Deciever, Realms of Death
Sabbath - Sign of the Southern Cross, Children of the Sea
Dio - Don't Talk to Strangers
Ozzy - Momma
Dokken - Alone Again
Kiss - I Still Love You
Slaughter - Fly to the Angels
Tesla - Love Song
Firehouse - All their famous ones
Crimson Glory - Lonely
Sacrerd Warrior - Sweet Memories
Dream Theater - Another Day (this song should still be played today on Jazz stations)
Hair of the Dog - Rescue Me
Brainstorm - Heavenly
All off the top of my head