Here are a handful that I think of as ones where Buffy either actively left Riley for Spike or her relationship with Spike was part of why her and Riley fell apart...
Out of My Head by AJ Hofacre
When you're evil, you're evil, right? Not necessarily. Not in William The Bloody's case, anyway. He's still as big of a Big Bad as they come -- but keep in mind that it's always been his passion that has gotten him as far as he is now. And maybe, just maybe, if that passion were tweaked a little bit... directed toward a certain person... We could see what Spike was really capable of.
Overcome by flibble
An unsatisfied Buffy comes across Spike while patrolling. This story is set in Season 4. Buffy and Riley are a couple but Spike is pre-chip.
Superstar Revamped by Kantayra
AU of the S4 episode 'Superstar'. With her friends brainwashed, Buffy must turn to an old enemy for help. However, soon she and Spike find themselves fighting a losing battle against the irresistible attraction between them.Are We Friends by slaymesoftly sequel
Just a Summer RomanceBuffy is alone for the summer, and she finds a surprising new friend and companion to help her stay amused. Feelings develop between Sunnydales two super blonds and life gets complicated...Battlestations by vicnoir
Buffy comes to terms with her feelings for Spike as they hunt for/run from a particularly nasty vamp from Spike's past. Spoilers: mid season 5-no spoilers to speak of.Whispers in the Dark by Ashlee
Battling a demon on a routine patrol one night, Buffy is seriously injured. When Giles realizes the full extent of the damage, he knows there is only one person who can help her get back to the Slayer she used to be.Who Are You? by zoegrace
Set late season five. Buffy and Riley are engaged to be married in two weeks when Buffy goes patrolling, gets hit in the head and loses her memory. Will amnesia allow her to see what's right in front of her?It All Started With Sex by Kari Mouke
It all started with sex. Riley did it badly. He knew it. They argued. Supposedly about Angel. Buffy said something she shouldn't. Not being who she was. Not being over a Hellmouth. Poof. Sex. Really, really good sex. The breaking up scene here couldn't be more clearly for Spike.
Bad Girlfriend Buffy by Kari Mouke
Morphology by cathmarchr
Accidentally claimed, Buffy and Spike must learn to live with each other.Uncontrollable by sweetprincipale sequels
Unmentionable and
Unknown (I think this is a WIP but sweetprincipale is an active writer so I would be surprised if it isn't continued -last update May)
et during S. 4"Where the Wild Things Are". Buffy and Spike end up being trapped in the Lowell House during the peak of the sexually driven "manifestation."