It's in everything. Obviously in things like sushi, but it's in shampoo, food, all sorts of things. Usually listed as alginate, agar, carrageenan, etc.
From the Wiki page: The food industry exploits the gelling, water-retention, emulsifying and other physical properties of these hydrocolloids. Agar is used in foods such as confectionery, meats and poultry products, desserts and beverages and moulded foods. Carrageenan is used in preparation of salad dressings and sauces, dietetic foods, and as a preservative in meat and fish products, dairy items and baked goods. Alginates enjoy many of the same uses as carrageenan, but are also used in production of industrial products such as paper coatings, adhesives, dyes, gels, explosives and in processes such as paper sizing, textile printing, hydro-mulching and drilling.
And it's in medicine, too.
http://en.wikipedia.org/wiki/Seaweed
And it's for dinner tonight here at home. We're having Spam Musubi and miso soup with seaweed threads.
Also used for making dental impressions...