While by no means exhaustive, Brands of the World does provide a nice sampling of logos, mascots and trademarks from around the world. It’s a good starting place for browsing around when you’re looking for branding inspiration or fodder for your next anti-corporate art action. Files are provided in EPS or AI. There’s a little trick to downloading though: it makes you click through the terms and conditions approval screen twice before you get to an actual download link.
Go forth and détourne, you gangsters of Photoshop.