What is a good vegetarian source of umami flavour? I'm looking for a source of the umami flavour to add to various dishes. However, since I became vegetarian I can't think of a good source for the flavour.

I used to use Worcestershire sauce, or Nam Pla (fish sauce). I can't think of a vegetarian source of the flavour in the same vein though. 

I've tried soy sauce, and it does work, but it imparts too much saltiness to the dish.
The Umami information Center has a list of Umami-rich foods along with natural concentrations of glutamate.  I've copied some of their list below (included some meats for comparison) in case the link disappears (concentration number is mg glutamate/100g food).  There's also some information at the above link about how to prepare the foods to maximize the umami taste.

Food/Concentration:


Beef/107
Tuna/188
Nori/1378 (not a typo)
Tomatoes/246 (the riper the better)
Fresh Shiitake Mushrooms/71
Dried Shiitake Mushrooms/150
Enoki Mushrooms/22
Soy Beans/66
Potatoes/102
Sweet Potatoes/60
Chinese Cabbage/100
Carrots/33