Wine+Art's thread on olive oils is great, but, to be honest, I find the Balsamic vinegar you use to be every bit as important to the salad or dish.
Unlike with olive oil, I'm not yet at the point where I am primarily comparing and contrasting the different qualities of differing Balsamics, I more frequently base my buying on overall quality. That said, I'd love it if people who have done balsamic tastings would chime in!
For me, my standards are the Bariani's CA Balsamic and <ducks for cover> the Trader Joes Gold Label (though the label is red) Balsamic. Both are reasonably priced ($15/500ml, $5/250ml respectively) and both are very good, with the Bariani a bit more pure tasting, but also a bit sweeter. I think I might actually prefer the Trader Joes. I guess the standard is the black bottle of Fini, which I like, but don't find worth the tariff. The white bottle of Fini is wonderful, but I go through so much balsamic, that it is way out of my price range.
I know I've had others I've really enjoyed, but I haven't gone exploring here nearly as much as I should have. So, what do you use?