My 16 month golden retriever has always been submissive and a little timid, but always very sociable. She gets to run off lead twice a day with a huge variety of other dogs and enjoys this tremendously. Over the past few days though, when we are walking through the woods (off lead) to the area where she meets most of the other dogs, she will stop and bark at  people/dogs coming towards us if she doesn't know them. She wags her tail at the same time and there is no aggression, but it seems to be a fear thing.
When they get close to her she's fine and usually starts running around with the other dog
I'm not sure how to deal with this behaviour but would like to 'nip it in the bud' particularly because if she does this to a non-doggy person they may find it a bit intimidating.
Any suggestions ?

is she barking at dogs/people she knows, or strangers?. if its friends its probably excitement and if strangers perhaps a warning,rising out of being unsure. i would try a few simple commands with rewards to distract her from the barking on these occaisions.I
have to do this most times i see a dog with mine to stop him from going over to them,or staring at them. it took a long time(longer than i care to admit!!!) but it did work in the end.

she barks at people/dogs she doesn't know and I think you are right in saying that she is unsure. What I have been doing so far is ignoring her and just carried on walking so that she sees that I'm not worried. Trouble is that she is usually running a bit ahead of me so meets them before me. I will take a squeeky ball in my pocket tomorrow and try that. What I don't want to do is to make an issue of it!

If it's people/dogs she doesn't know, it is possible she is wary of them. I would recall her, put yourself between her and the other dogs/people to show her that you aren't worried about them, and distract her with a toy or treat. (she might be less wary off lead, but if you cannot guarantee she will stay to hell, put her on)

One WSD does this with people who dont have dogs and if he doesnt know them.  I can tell by the alarm bark he gives off.  I just recall him back to me, make him sit so they walk by and then play afterwards to make it a good thing.