A 3rd Level spell castable by literally *every* spellcaster worth noting in PF. This Immediate-Action casting spell is really, really nice. It could be argued that it may not have a properly allocated casting level (3rd), but it looks about right for what it can do. Plus, it promotes nice teamwork synergy, has cool/powerful combo possibilities and can help cover/fix weaknesses in a party's overall composite with multiple spellcaster types.
If you backport it to 3.5, there's tons of fun things you can do with this spell. You can stick it on an Eternal Wand or have your Familiar cast it on you via Imbue Familiar with Spell Ability spell.
Plus, with some spell research shenanigans, a spellcaster could easily have the *4* +1 metamagic feats that the spell simulates ... be changed to a different array of +1 metamagic feats. Or even the *2* additional +3 metamagic feats for the rarely-used higher-end option within the spell could be changed to metamagic feats like Chain Spell or Echoing Spell.
Some neat things a party can do with Collaborative Thaumaturgy are:
1. Help another spellcaster "stealthcast" a Silent/Still spell that ... possibly in a group of people where there's a sensitive situation going on.
2. Can help imbue a friend's spell with Silent Spell if he's under a magical silence effect.
3. With impeccable timing and a Lesser Chain Spell Rod, a spellcaster could Chain this spell to a boatload of other spellcasters to Extend some key spells all being cast simultaneously.
Etc. and so on ...