Falling, by Slyark -- a Carter/Luka in the Congo fic. This can be found at www.errealmofslash.com and you can also search for it on the internet.
Consequences, by Julie -- this is a work in progress, and is an excellent psychological study of the two men. It's only available at ERROS (er realm of slash), but is not yet back in the archive. Keep checking, because the archive is being updated daily.
To Dream, Perchance To Love, Summer In The City, Hearts In Darkness, Crossroads and For Such As These, by Kathryn "Prozac" Parsons. Again, usually at ERROS, and will be back up soon. These are epic instllments of a series featuring the two doctors.
There's also a Dave/Luka fic that has an episode with Carter in it, lots of hotness:
Waiting For A Valentine, by Ceindreadh. At ERROS and online.
There are some others at ERROS, but they're mine, and I'm not too sure about recommending my own fics :-)