The Wall Street Journal has a short but interesting piece on the world of Apple t-shirt collecting. There are two types of Apple t-shirts. The first are those shirts that the general public have access to via giveaways at Apple retail stores, such as the t-shirts at an opening of a new Apple Store. The second type are t-shirts that are only available to Apple employees -- often at the retail level.
As the WSJ article points out, Apple strictly forbids employees from selling, donating, or giving away the shirts. However, that's doesn't stop them from showing up on eBay from time to time. I myself have a small collection of shirts from my years at Apple. You can check a few of them out in the gallery below.