Well, aside from the random Buffy-verse-like lines that pop up --
"They make me feel manly."
and such --
I can't think of any off the top of my head -
But I *am* waiting for him to use:
"Well, this is just... neat." -- Spike - "Halloween"
One of my faves --
But, my all-time fave Spike line that I would *love* to see in a book (preferably w/ Harry speaking it) --
Spike to Xander in "Triangle" -- "Yeah, I could do that, but I'm paralyzed with not caring very much."
(let me know if you want context)
Other BTVS connections --
Well, I guess there is the Angel connection - he's a detective (sort of) -- tho he's not a vamp w/ a soul (heaven forbid) - he's an 'outcast' of sorts --
Highlander II
Highlander II's Medium-sized Kingdom
"Yeah, I could do that, but I'm paralyzed with not caring very much." -- Spike