Lemon balm substitutes adequately for lemongrass. It can be a pleasant vegetable seasoning. It can also be nice in fruit or green salads, in small quantities, and I've even used it in baked goods. It's an OK additive to teas and liqueurs, but I don't care for plain lemongrass concoctions because they tend to have too much of that furniture polish quality. I did make an interesting lemon balm sorbet once.
I have grown it and other herbs for years. The trouble is, all of the good culinary uses for lemon balm call for sparing amounts, and I always have massive amounts. Although it is a mint relative, in my garden, it's much more vigorous than the mints I've grown. Most mints like cooler, damper conditions; melissa spreads rapidly in full sun.
I'm always willing to try new uses for herbs, but so far the best use for lemon balm, in my experience, is as potpourri.
Anyone have any more ideas for finding tarragon?