Addon to assist with mage's Spellsteal
What are addons that assist with a mage's Spellsteal. Something that tell a mage there is a buff or spell that can be stolen from a mob.
I would recommend several add-ons in addition to MageNuggets which Ward mentioned, both of which are updated regularly.
SpellStealer
From the Curse page:
This simple addon pops up a small frame with a list of stealable buffs on your current target, it will only show up when the target is an enemy and has buffs you can steal.
You can download it here. This add-on is more focused on just SpellSteal prompts, whereas MageNuggets has many Mage specific functions.
Power Auras
This is personally my best recommendation, as Power Auras is good for many other things, so chances are you can leverage the functionality for many things, if you aren't already! Using PowerAuras, there is a "Stealable Spell" option that you can set for a given aura.
From the Curse page:
This addon was created to provide visual cues (auras) when you gain buffs, debuffs and many more. Very useful for shorts buffs or debuffs, it allows you to draw in the center of the screen, or around your character, very customizable visual effects, rather than having to look at or mouseover buff/debuff icons/actionbars.
You can download it here.
There is an active wiki for Power Auras maintained here, and you can find many detailed tutorials on the web for Power Auras, but I would recommend this one, which is for a slightly older version but the basics still apply. It may look daunting with all those options at first but it can be very powerful, even more so than the new in-game "Spell Alerts", which were basically a Blizzard implemented copy of Power Auras due to the popularity of the add-on.
Look into mage nuggets, you can find it on curse. It claims
SpellSteal Monitor: Displays stealable buffs on enemies that you have targeted. Some buffs were just ment to be stolen and this helps you find them. If you don't have Spellsteal on your bars, you will. Notifies you when you successfully steal something via combat text and the chat console and tells you what you stole in a linkable format.
I havent personally used it though.
The default target frame will highlight stealable buffs with a bright outline. That's all I use and it seems to work well.