Here is some more information on some of the more popular quest rewards.

Act I: Quest 1: Den of Evil
+1 Skill Point, 1 Free Respecialization (reset stat points/skill points)

Act I: Quest 5: Tools of the Trade (Imbue)
Imbue creates a new rare item out of the base item you want to Imbue. The stats of the base item do not matter as it is only used to determine what type of rare item will result from using Imbue. Also, the quality of the item used (Superior, Normal, Damaged, etc.) does not affect the new item. So don't waste your time looking for a Superior Item or for specific Defense because it doesn't matter. Any properties of the old item are lost when the new rare item is created. Your character level determines what magical properties your new rare item can have, so characters with higher levels have more, but not always better, options available. Consult the Magic Prefix and Suffix pages for a listing of Magical Bonuses and levels. Since the magical properties on the item created are random, you might not always receive a 'good item' from Imbue.

It's often best to save your Imbue and use it on an Exceptional or Elite Item.

Act II: Quest 1: Radament's Lair
+1 Skill Point / 10% Reduction in prices

Act III: Quest 1: The Golden Bird (A Jade Figurine)
+20 health points (permanently)

Act III: Quest 4: Lam Esen's Tome
+5 Stat points

Act IV: Quest 1: The Fallen Angel
+2 Skill Points

Act IV: Quest 2: The Hellforge Reward
Classic: 4 Gems - 1 Normal, 2 Flawless, 1 Perfect. All gems types are chosen randomly and include skulls.

Expansion: The expansion includes the 4 gems as you would get in Classic and 1 randomly chosen rune per difficulty from the range below.

Normal: El - Amn
Nightmare: Sol - Um
Hell: Hel - Gul

Act V: Quest 1: Siege on Harrogath (Add Sockets)
You cannot use the Add Socket Quest reward on Socketed Items.

You can only add Sockets to items that allow Sockets.

Specifically, it does not matter in which difficulty the quest reward is used. The outcome is the same regardless.

Larzuk adds the maximum number of sockets to regular items, based on: Base Item, Item Type, and item's (creation) level. The latter, which is often referred to as the 'ilvl', is determined by the source that dropped the item (monster's level if dropped from a monster). It makes no difference if the item is low (Cracked, Crude, Damaged, Low Quality), normal or high quality (Superior). Ethereal does not affect the outcome either. Again, only base item, item type and ilvl matter. NOTE: Because of the dependency on ilvl, your item may get fewer sockets than the inherent maximum ('Sockets') which is listed on the [Weapons and Armor] pages, if the ilvl is not sufficiently high. Use ilvl > 40 if you want the inherent maximum sockets in general.

Magic Items will receive 1 or 2 Sockets with probability of either assuming that they are possible given the rules for normal. So you'll either get 1 Socket or 2.

Set Items, Rare Items, Unique Items, and Crafted Items add 1 Socket (1 Socket Max).

Act V: Quest 3: Prison of Ice
All Resistances +10 (Scroll of Resistance), rare class-specific item for the class played (e.g. an Amazon Bow / Javelin / Spear if you play an Amazon).

Act V: Quest 4: Betrayal of Harrogath (Personalize)
Personalize adds your character name to an item, such as "Stormchaser's Frostburn Gauntlets". The item is also repaired and recharged. Keep in mind that items might lose trade value with a name on it if you decide to trade them. Some people don't want to look at other people's names on their items, especially if their name is "iiiiiiiiiiiiiiiiiiiiii". Any Armor and Weapons are nameable (except Quest Items and Throwing Potions). Other items cannot be personalized.

Act V: Quest 5: Rite of Passage (Experience Boost)
You cannot get credit for this quest unless your character Level is at least 20 in Normal, at least 40 in Nightmare and at least 60 on Hell difficulty. The exact maximum experience points gained are as follows,

Normal: 1,400,000 (1.4 million)
Nightmare: 20,000,000 (20 million)
Hell: 40,000,000 (40 million)

This is however limited to at most 1 level-up. If you would get 2 level-ups or more from the above, you still only get 1 level-up and are the exact same amount of experience points from reaching the next level as you were from reaching the current level before you completed the quest. In other words, completing this quest "too early" (e.g. at level 20 in Normal) amounts to wasting experience, since you gain less than maximum. Using the table of [Experience Levels], you can determine minimum levels that guarantee a level-up as well as the maximum experience.

NOTE: The values are fixed, and the usual rules for calculating experience in multiplayer games as well as experience boosters (Annihilus, Ondal's Wisdom and/or an Experience Shrine) do NOT apply here. Keep this in mind before taking on the Ancients in an 8-player game, as they may prove very hard to defeat.