I usually pass them notes. So do most GM's I have played with. If there is some complex information I can anticipate someone maybe getting, I can write it up in advance, to speed things up during play. I may even write a few versions of some notes, or some notes that are useful in a variety of situations that may come up, possibly even will "fill in the blank" spots. e.g.:
You notice a ________ figure _______ who is _________.
If the info is more complex or wants interaction, I'll get with them for a quick whisper or possibly take them in another room briefly.
In some games, each player has a notebook for writing notes in, and they get passed to the GM and back.
Also, when using computers, I have prepared some texts that can be sent via electronic message. Nowadays, this could also be used with people's phones... though I haven't experimented with that, and would tend not to because I'd rather not have people relating to their devices during play.
People I have played with have generally really enjoyed this type of play. Many times it has enabled some very interesting situations where the players are generating a lot of what is happening. What people know or don't know can be very interesting and important and fun to work with, and even with great players, it can be helpful and more fun to not have the same view and knowledge as everyone else. Of course, it can also sometimes become tedious or slow things down, and might not always be necessary or desirable.
In the examples you gave:
Ex 1. A rogue loots a corpse but doesn't want to split the gold. How
does he let me know he's looted the corpse without announcing it and
having all the other players hear him? Sure the PCs didn't hear him
announce it, but the players did and they'll react in some way (i.e.
it'll spark their reaction to do the looting next time, or they'll
hold a grudge).
The player would generally pass me a note. Sometimes my players have pre-prepared notes themselves. Sometimes they even pass me notes with trivial things or nothing "just to keep the other players from metagaming other note passing". Some players have also sometimes established some signals they can give me to indicate they are doing some pre-arranged action, so no note is needed. e.g. The rogue player above might have told me he will fiddle with the eraser on his red pencil when he wants to quickly pre-loot a body.
Ex 2. My group is travelling through the woods and the Ranger asks if
she can make a check to see if there are nearby goblins. Rolls high
enough to determine that there are some that have been there recently.
No one else would have noticed. Do I just tell the Ranger, or do I
tell the whole group because it's likely what the ranger is to do
anyways.
Your call. I would look at how the players are deployed at the moment, and whether the Ranger is clearly going to tell them, and whether he can do without yelling. If he would almost certainly let them know, I'll just say so, but if I do decide to tell all the PC's, and the Ranger decides not to tell them for whatever reason, then I need to expect the other players to roleplay their lack of information. So I weigh that whole situation when making that kind of decision. I've been through that sort of thing enough that it's pretty quick for me to decide and tweak to suit the situation.
Ex 3. My players meet a Dragon yet only one of them speaks Draconic,
and the dragon doesn't speak Common. Maybe the player would want to
translate only half truths to the party, so how can I tell just him.
In that case I would use notes, and/or take the player to the side of the room, where I can whisper to him the Draconic content, while more loudly saying what is going on to the other players.
