For those new to the Humble Bundle, it started out as a biweekly special on a pack of independent computer games. As it's grown, it's branched out into ebook bundles and, as featured in this thread, RPG bundles. The common motif is that there's a degree of "pay what you want" with certain extra content getting unlocked based on either meeting the current community average or paying a certain dollar amount. The RPG bundles seem to start at a hard floor of $8.95 for a core collection, but otherwise they follow the same plan. Additionally, you can adjust where your money goes: charity, authors, and the HB crew. It's a really neat way to introduce people to neat stuff and raise money for charity.
The permanent Bundle of Holding URL is
http://www.bundleofholding.com/index/current.