Recently someone somewhere in the spuffy tag said something about how Florence from Florence + the machine is a BtVS fan. These songs I am semi sure are about Spuffy:
How Big, How Blue, How Beautiful (yes, the song is about Spike's eyes and Buffy grieving him)
Kiss with a Fist (season 6)
The Bomb (someone in the Spuffy tag already talked about this)
Buckle (Spike's perspective on the Spuffy relationship from season 5 to 7)
You've got the Love (Buffy's perspective of the Spuffy relationship in season 6 and 7)
Shake it out (Buffy season 7)
Which Witch (Mainly vibes but it just reminds me of the balcony scene from season 6)
Never let me go (Spike, season 7, especially when he's insane)
Everybody scream (season 6)
Cosmic Love (Spike, season 5 onwards)
Ship to wreck (I can't even say whose perspective when it is, it's just there)
Sympathy Magic (Buffy post resurrection)
No light, no light (both season 6 and post season 7. I don't accept questions on this one as I can't explain)
Delilah (Sprucilla, specially "Lover's walk" as Spike is still hung up on Dru but aware of his connection to Buffy)
Witch Dance (season 6, Smashed and onwards)
Breath of Life (full Spuffy, "Something Blue" but also seasons 6 and 7)
Dream Girl Evil (Buffy's assumptions in season 6)
Hunger (Your honor, it's them. They are both so desperate for love. Start to finish)
Howl (listen to the song. Tell me it's not them. It's them)
What kind of man (Bangel. Bangel Hate to be specific)
Heavy in your arms (season 6)
Choreomania (it just screams "once more with feeling")
Kraken (Spike pre soul, kinda giving "Seeing Red" and also "Fool for Love")
Rabbit Heart (raise it up) (season 7 and season 5. Also season 2. Don't question me)
Sky full of song (Ats season 5)
All this and Heaven too (Spike, from "Fool for Love" onwards)
Light of Love (Buffy's season 6 healing arc, Spike freshly ensouled)
Hardest of Hearts (Spike about Buffy)
I'm sure there are more, but it's late, so that's all on my side!