The answer to this question is as broad as the horizon. You can find anything that you would normally find in a home or a business in a storage unit. Although most storage units contain a lot of common items, if you buy enough units, you are bound to find some real treasures in some.
I have personally bought that contained rifles, shotguns, pistols, mint proof sets, vintage guitars, high-end tools, stamp collections, coin collections, sports card collections, motorcycles, a antique Coke machine, high-end appliances, over 5K in new Gucci & Versace shoes, luxurious furniture & antiques. One of the best storage units that I bought contained a vintage guitar & a safe which contained gold, silver, cash, WWII & Civil War memorabilia, an antique revolver and an antique pocket watch collection. Although this doesn't happen often, you can find some amazing & quite valuable items in storage units.