Nectar Wine Lounge or Cav Wine Bar
I am a huge fan of CAV and go there regularly. I have only been to Nectar in the Marina once (there is also one in Burlingame) and I thought the printed descriptions of the wines were a little simple for my taste, but I brought a new wine drinker there and she understood and appreciated them. As a regular at CAV, I have learned a lot about wine from the staff and am very used to Pamela's wine notes on the menu. I also find the food at CAV to be consistently delicious.
Can't speak to Nectar (I cannot appreciate the Marina though I've tried), but go to CAV often. Unlike most wine bars, they've actually got great food -- thank you, Chef Christine! We always order the pork tagine when it's cold out and we need a rich, warming meal. If it's fall or winter, they will likely have the venison chops, which are divine. For dessert, the chocolate cake with the melty middle is really yummy. The cheese plate of three cheeses is also fantastic. They drizzle a strip of honey lengthwise down the middle and space the cheeses out. Each comes accompanied with a little friend: quince gelee squares, marcona almonds, filberts,etc.
As far as wine, I find that they often have a slew of very interesting Alsation or German whites that do not make an appearance at many other venues. (I don't know if they're still doing that thing on the last Saturday of the month, the afternoon cookout? The one time I went it was a bbq with homemade sausages and they poured a ton of German and Alsation whites and also roses.) They also make an incredible effort to include wines from around the world and not necessarily the usual places. Croatia and Hungary have long wine making histories, but I don't see those on a lot of other tasting menus. What I like about the place is that the staff does not cater only to older customers. They have always been courteous and knowledgeable, and will spend time with you talking wine (unless it's superpacked). I have learned a lot about wines from Tadd.
CAVeat: if you go near the end of the month, they will likely have run out of some of their more popular wines. It has happened to me a couple of times. But they can point you toward something similar. And let's face it, sometimes when you can't get something because they've run out, it makes you want it a little bit more, and that's kind of a fun feeling.