Extend Spell (PHB, +1)+++ - good for buffing or summoning spells
Empower Spell (PHB, +2)+++ - for buffing or damage spells, like Searing Light
Silent Spell (PHB, +1)++ - mainly for Dispel Magic
Reach Spell (DoF, +2)++ - for ranged cure/inflict spells or Slay Living, Harm/Heal
Still Spell (PHB, +1)++ - good if you're grappled
Heighten Spell (PHB, +x)++ - To raise the DC of your Spells, e.g. Hold Person, Halt Undead...
Sanctum Spell (DoF, +0)++ - for NPCs, DC +1
Quicken Spell (PHB, +4)+ - Harm + Quickened Inflict Moderate Wounds...
Persistent Spell (FRCS, +4)++ - Questionable Ruling, good for many spells, like Detect Thoughts, Detect Magic, Divine Favor, Detect Evil...
Not metamagic, but very useful:
Spell Girding (FRCS)++ - Make your spells harder to dispel (DC +2)