Pick-a-Pack is a casual limited Magic: The Gathering format that allows players to draft by combining packs from many sets. 
In the Pick-a-Pack draft format, players draft just as they would during a normal booster draft with one change: instead of using three booster packs from the most recent Magic block, players are allowed to use packs from any Magic set. That means the player to your left could be using a pack each of Darksteel, Ravnica and Ice Age while the player to your right is cracking two packs of Shadowmoor and one pack of Tenth Edition!
A variant of Pick-a-Pack is Pack Draft. Each player brings three booster packs to the draft, and those packs are collected into a common pool. Before the players draft the cards out of the boosters, they draft the boosters themselves! Once all the packs are picked, the actual booster draft begins. Players can open any of their three boosters first.
You can use Pick-a-Pack in nearly any draft format, such as Solomon Draft or Winston Draft, thus combining that other format's rules with the ability to use any pack from Magic history!