There are sub-series in irena_adler's 'Watson' (http://irena-adler.livejournal.com/3840.html) series where Charlie initiates a short-term relationship with David (Balancing the Equation, A New Gravity) and begins a long-term relationship with Colby (It Changes Everything, It Changes Everything: Charlie).

Also, off the top of my head, is penguingal and schnaucl's collaboration series called 'Triangulation' (http://www.livejournal.com/tools/memories.bml?keyword=Triangulation-verse&user=penguin_fics&sortby=des). The first part is called 'Breaking Point'(Don/Charlie).

If I think of anything else, I'll let you know. :)