The Buff Deck for Pathfinder is a cheap but extremely handy way to remember all of those temporary little bonuses that are otherwise easy to forget about during an encounter. The deck consists of 54 cards, 49 of which each contain the effects of a spell, ability, or action that gives a character a benefit to attacks, AC, or saves. All of the usual buffs from the Core Rulebook are represented (like haste, bless, inspire courage, total defense, barbarian rage, etc.) but there are also several buffs from the Advanced Player's Guide (like alchemist's mutagen, cavalier's banner, etc.). The face of each card is divided into six sections: the title of the buff, its duration, its source (including book and page number), a list of effects, a table showing the type and extent of bonus to attacks and AC, and a table showing the type and extent of bonus to saves. It was an excellent choice by the designers to include the type of bonus (e.g., morale, deflection, enhancement, etc.) so that it's easy to see which buffs do and do not stack with each other. The presentation is attractive and easy to read even across the table. In addition to the 49 pre-made cards, there are 5 cards with the same formatting but no content so that a GM or Player can add a buff commonly used that isn't in the deck--for example, I added a card about the effects of a ranger's favored terrain when I regularly had a ranger PC at the table. This might seem like a little thing, but it adds to the deck's longevity given the vast number of spells and abilities that come out every year for Pathfinder.
As for downsides, I don't have much. Each card has the same artwork on the back, and it's kinda cartoony. Some of the buff cards aren't precisely accurate: the card for protection from evil doesn't mention the added defence against mind-control effects or versus summoned creatures, the prayer card leaves out the fact that it imposes a penalty on enemies (which is technically a debuff, I guess), the invisibility card doesn't mention the 50% miss chance or +2 bonus on attacks, etc. Still, the vast majority of cards accurately summarize the effects of a given buff.
I've used the Buff Deck for several years now, propping up a card in effect so that everyone at the table can see it. Although not foolproof (even the cards can be forgotten when encounters get harry), they are quite useful and speed up game play. What more could you want?