Others that people should check out:
82games.com
basketballvalue.com
hoopsstats.com
hoopdata.com
I'm not aware of any site where you can look at stats of one player while another player is in the lineup. You can look at how a team does in such situations, though, on basketballvalue.com or on nba.com, among other places.
Actually, what do you usually use those sites for?
82games.com -- a little bit of everything. on/off court data, lineups, production and opponent's production at various positions, eFG% on jump shots, etc.
basketballvalue.com -- This is best for up-to-date lineup information, as in what lineups are most / least efficient, etc.
hoopsstats.com -- You can find most stuff elsewhere, but they have interesting per-quarter stats. See
Link.
hoopdata.com -- Good for seeing a player's efficiency from various spots on the court (i.e., 10-15 feet, etc.)