Best file manager: GoodReader
Many people really wish the iPad had a shared file system, like a PC or Mac. It doesn't, but GoodReader can give you much of the file manager you want. It provides a central file repository for files you transfer via Wi-Fi, various storage services, iTunes, and the Open In facility used by many iOS apps (such as Mail). As its name implies, GoodReader lets you read many file formats, including several not supported by iOS's naive QuickLook facility. Plus, it unzips file archives, so you don't need a separate utility for the task.