I love squeezing the rules until something interesting falls out, and here's what I've been thinking recently.
Getting spells known of a higher spell level than your highest available spell slot is easy:
*Clerics and Druids know all spells of all spell levels they are able to cast divine spells from (Source: the Magic chapter of the Core). So, having a high-level divine Spell-like Ability gets them every spell of that level as a spell known.
*Oracles can take the Haunted curse and accelerate the curse with a Favored Class Bonus, getting Telekinesis as a spell known at level 7.
*Ancient Lorekeeper Oracles can add wizard spells known of a level beyond their ability to cast.
*Expanded Arcana (feat from APG) will let you add one spell known, with no level restriction.
*Wizards can just straight up scribe any wizard spell they come across.
*Abilities that let you add spells "lower than the highest level spell you can cast" can be tricked. Favored Class abilities, spontaneous caster retraining, cross-class pilfering like the Magaambyan Arcanist or the Pathfinder Savant (who can actually just scribe an 8th level spell from any class in his spellbook without using any tricks). This is very useful in and of itself.
But what do you do with your high level spells known? None of the immediately obvious tricks will let you actually cast them: Arcane Bonded Object and Spirit Ranger both stipulate spells you're actually capable of casting. IIRC, Heighten Spell + Metamagic cost reducer has been patched, so that trick is out.
The Shadowcasters Shadow Spells ability seems to be a winner. " He may prepare a number of additional spell levels of spells equal to the level of the highest-level wizard spell he can cast." For this to be abusable, a SLA needs to qualify. Let's quote an official post from the Pathfinder Design Team:
"The same rule should apply for all creatures with spell-like abilities, including PC races: the creature's spell-like abilities are presumed to be the sorcerer/wizard versions"
As an example, let's take an Elf Wizard with the Dreamspeaker alternate racial trait and 15 Cha. He can cast Dream, as a 5th level sorcerer/wizard spell, 1/day. That means he can prepare 5 levels of spells using Shadow Spells. If he can get his hand on a 5th level spell to scribe it, that means that this 1. level wizard can cast an actual 5th level spell. Of course, he's also an Enchanter (Controller), and starts casting Gate and Blood Money Wishes once he's level 8.
Other possible exploits:
Collaborative Thaumaturgy will let you maximize or widen a spell. With 3 assistants. I see this more as a trick for expensive permanent spells than an adventuring activity.
Metamagic Mastery Unfortunately, you can't get this AND the delicious 9th level spell of an 8th level Enchanter. But there are still ways to squeeze in a spell slightly better than what you'd otherwise be able to cast.