Source: Complete Arcane
You can use a spell-like ability at its maximum effect.
Prerequisite: Spell-like ability at caster level 6th or higher.
Benefit: Choose one of your spell-like abilities (subject to the restrictions below) to use at maximum effectiveness up to three times per day (or the ability's normal use limit, whichever is less). All variable, numeric effects of the spell-like ability are maximized, dealing maximum damage, curing the maximum number of hit points, affecting the maximum number of targets, and so on. For example, a 10th-level warlock's maximized eldritch blast deals 36 points of damage three times per day. Saving throws and opposed checks (such as the one you make when you cast dispel magic) are not affected, nor are spell-like abilities without random variables.
An empowered maximized spell-like ability gains the benefit of each feat separately (getting the maximum result plus one-half the normally rolled result). For example, a fire mephit's empowered maximized scorching ray would deal 24 points of damage plus one-half of 4d6 points of damage.
The spell-like ability you wish to maximize can be chosen only from those abilities that duplicate a spell of a level less than or equal to 1/2 your caster level (round down), minus 2. For a summary, see the Caster level to Empower column in the table on page 304 of the Monster Manual.
Special: This feat can be taken multiple times. Each time, you apply it to a different one of your spell-like abilities.