There are surely other Valentine episodes I've never seen, and probably some I just don't remember. This will just be things I either remember, or else probably saw, since I watched the series in question. Oh, and while the holiday may play a particularly prominent role in many of the episodes listed here, sometimes it's only a very minor aspect of the episode's plot.
3rd Rock from the Sun
Season 5: Dick Puts the 'Id' in Cupid (Amazon; IMDb)
30 Rock
Season 1: Up All Night (Amazon; A.V. Club; IMDb; Wikia; Wikipedia)
Season 3: St. Valentine's Day (Amazon; A.V. Club; IMDb; Wikia; Wikipedia)
Season 4: Anna Howard Shaw Day (Amazon; A.V. Club; IMDb; Wikipedia)
Season 6: Hey, Baby, What's Wrong (Amazon; A.V. Club; IMDb; Wikia; Wikipedia)
American Dragon: Jake Long
Season 2: The Love Cruise (IMDb; Wikia)
American Dreams
Season 1: Heartache (IMDb; Wikia)
American Housewife
Season 1: Time for Love (Amazon; IMDb; OHS Wiki; Wikia)
Arrested Development
Season 1: Marta Complex (Amazon; A.V. Club; IMDb; TV Tropes; Wikia)
The Big Bang Theory
Season 3: The Large Hadron Collision (Amazon; A.V. Club; IMDb; Wikia)
Season 6: The Tangible Affection Proof (Amazon; A.V. Club; IMDb; Wikia)
Season 7: The Locomotive Manipulation (Amazon; A.V. Club; IMDb; Wikia)
Season 9: The Valentino Submergence (Amazon; A.V. Club; IMDb; Wikia)
Black-ish
Season 1: Big Night, Big Fight (Amazon; A.V. Club; IMDb)
Bob's Burgers
Season 3: My Fuzzy Valentine (Amazon; A.V. Club; Hulu; IMDb; OHS Wiki; TV Tropes; Wikia; Wikipedia)
Season 5: Can't Buy Me Math (Amazon; A.V. Club; Hulu; IMDb; OHS Wiki; TV Tropes; Wikia; Wikipedia)
Season 6: The Gene and Courtney Show (Amazon; A.V. Club; Hulu; IMDb; OHS Wiki; TV Tropes; Wikia; Wikipedia)
Season 7: Bob Actually (Amazon; A.V. Club; Hulu; IMDb; OHS Wiki; TV Tropes; Wikia; Wikipedia)
Season 8: V for Valentine-detta (Amazon; A.V. Club; Hulu; IMDb; TV Tropes; Wikia)
Season 9: Bed, Bob & Beyond (Amazon; A.V. Club; Hulu; IMDb; TV Tropes; Wikia)
Season 11: Romancing the Beef (Amazon; A.V. Club; Hulu; IMDb; TV Tropes; Wikia)
Season 12: Ferry On My Wayward Bob and Linda (Amazon; Hulu; IMDb; TV Tropes; Wikia)
Boy Meets World
Season 5: First Girlfriends' Club (Amazon; IMDb; Wikia)
Buffy the Vampire Slayer
Season 2: Bewitched, Bothered and Bewildered (Amazon; A.V. Club; IMDb; TV Tropes; Wikia; Wikipedia)
Castle
Season 5: Reality Star Struck (Amazon; IMDb; Wikia)
Cheers
Season 9: Sam Time Next Year (Amazon; IMDb)
The Cleveland Show
Season 4: Here Comes the Bribe (Amazon; IMDb; OHS Wiki; Wikia)
Code Lyoko
Season 2: Saint Valentine's Day (IMDb; TV Tropes; Wikia)
Community
Season 1: Communication Studies (Amazon; A.V. Club; IMDb; TV Tropes; Wikia; Wikipedia)
Season 2: Early 21st Century Romanticism (Amazon; A.V. Club; IMDb; TV Tropes; Wikia; Wikipedia)
Cybill
Season 1: Call Me Irresponsible (Amazon; IMDb)
Season 3: Valentine's Day (Amazon; IMDb)
Desperate Housewives
Season 1: Love Is in the Air (Amazon; IMDb; Wikia; Wikipedia)
Dharma & Greg
Season 1: Dharma and Greg's First Romantic Valentine's Day Weekend (IMDb)
Season 2: See Dharma Run (IMDb)
Season 3: The Trouble with Troubadour (IMDb)
Dr. Quinn, Medicine Woman
Season 3: What Is Love? (Amazon; IMDb; Wikia)
Fresh Off the Boat
Season 2: Love and Loopholes (Amazon; A.V. Club; Hulu; IMDb; Wikia)
Season 5: Cupid's Crossbow (Amazon; Hulu; IMDb; OHS Wiki; Wikia)
Friends
Season 1: The One with the Candy Hearts (Amazon; A.V. Club; IMDb; Wikia)
Season 8: The One with the Birthing Video (Amazon; IMDb; Wikia)
Futurama
Season 2: Put Your Head on My Shoulders (Amazon; A.V. Club; IMDb; Infosphere; TV Tropes; Wikia; Wikipedia)
Season 4: Love and Rocket (Amazon; A.V. Club; IMDb; Infosphere; OHS Wiki; TV Tropes; Wikia; Wikipedia)
Gilmore Girls
Season 6: A Vineyard Valentine (Amazon; A.V. Club; IMDb; Wikia)
Glee
Season 2: Silly Love Songs (Amazon; A.V. Club; IMDb; TV Tropes; Wikia; Wikipedia)
Season 3: Heart (Amazon; A.V. Club; IMDb; TV Tropes; Wikia; Wikipedia)
Season 4: I Do (Amazon; A.V. Club; IMDb; TV Tropes; Wikia; Wikipedia)
Grimm
Season 3 webisode: Love Is in the Air
Home Improvement
Season 1: Baby, It's Cold Outside (Amazon; IMDb; Wikia)
Season 6: A Funny Valentine (Amazon; IMDb; Wikia)
How I Met Your Mother
Season 5: Rabbit or Duck (Amazon; A.V. Club; IMDb; TV Tropes; Wikia; Wikipedia)
Season 6: Desperation Day (Amazon; A.V. Club; IMDb; OHS Wiki; TV Tropes; Wikia; Wikipedia)
Season 7: The Drunk Train (Amazon; A.V. Club; IMDb; OHS Wiki; TV Tropes; Wikia; Wikipedia)