There are currently 17 different mustards living in my fridge, and I couldn't pare it down. I need every single one. One of the greatest websites is the Mustard Museum of Mount Horeb, Wisconsin. (where else?) You can shop for as many mustards as you like, and the mustard museum gift packs them for you. It's simple and makes a great gift. You don't have to tell people how to use it (like truffle oil), and... It's mustard! Everybody likes mustard!
My pet mustard is the bright green Edmond Fallot Tarragon Dijon, mildly spicy and packed with fresh herb flavor. It tarts up a quick cream sauce, is essential for my slow-baked salmon, and belongs on every roast beef sandwich. And I get my Pudliszki Musztarda Sarepska to put on kilbasa to remind me of home. It's a spiced mustard, along the lines of Dijon...but more subtle and perfect for SAHY-sage.