PickUp, an ability that a few Pokémon have, allows for the Pokémon in question to have a 10% chance of finding an item after battle. In RSFRLGCXD, these were a set few items with varying rarities. diamondpearl however brought in the inclusion of different items at different levels. Diamond & Pearl follows this inclusion with their own items. Below are the Pokémon that can learn this Ability. Click them to go to their Pokedex entry
Meowth Aipom Teddiursa Phanpy Zigzagoon
 
Linoone Pachirisu Ambipom Munchlax  
 

Now, each Set of Levels have varying items, with some items being rare for the lower levels but common for the higher levels. Hover over the items for their name:

Lv. Common Uncommon V. Unc. Rare V. Rare
1 - 10 Potion Antidote Super Potion Great Ball Repel Escape Rope Full Heal Hyper Potion Ultra Ball Revive Nugget
11 - 20 Antidote Super Potion Great Ball Repel Escape Rope Full Heal Hyper Potion Ultra Ball Revive Nugget King's Rock
21 - 30 Super Potion Great Ball Repel Escape Rope Full Heal Hyper Potion Ultra Ball Revive Rare Candy King's Rock Full Restore
31 - 40 Great Ball Repel Escape Rope Full Heal Hyper Potion Ultra Ball Revive Rare Candy Dark Stone Full Restore Ether
41 - 50 Repel Escape Rope Full Heal Hyper Potion Ultra Ball Revive Rare Candy Dark Stone Light Stone Ether White Herb
51 - 60 Escape Rope Full Heal Hyper Potion Ultra Ball Revive Rare Candy Dark Stone Light Stone Awakening Stone White Herb TM 44 - Rest
61 - 70 Full Heal Hyper Potion Ultra Ball Revive Rare Candy Dark Stone Light Stone Awakening Stone Full Restore TM 44 - Rest Elixer
71 - 80 Hyper Potion Ultra Ball Revive Rare Candy Dark Stone Light Stone Awakening Stone Full Restore Max Revive Elixer TM 01 - Focus Punch
81 - 90 Ultra Ball Revive Rare Candy Dark Stone Light Stone Awakening Stone Full Restore Max Revive Elixer TM 01 - Focus Punch Leftovers
91 - 100 Revive Rare Candy Dark Stone Light Stone Awakening Stone Full Restore Max Revive Elixer Max Elixer Leftovers TM 26 - Earthquake