I dunno how dirty it is, but there's the secret page trick, wherein you doodle in your spellbook, and then use secret page to make your doodles be whatever spells you want. Every spell ever, for free at level 5. And your professors told you doodling in your notebooks was a waste of time!
Astral projection was mentioned by Lycanthromancer for its practical invulnerability available as early as 9th level for Wizards. However, it goes beyond just that for abuse. Since you get a fresh body each time you manifest, you get all-new copies of your gear, including charged items, so you never need to buy more than one of whatever charged item you want. Also, since your real body never takes damage while you're astrally projected, and you make a copy of your real body whenever you manifest, you get infinite free healing whenever you need it.
Expanding on the abuses one can pull with planar binding, remember that racial spellcasting and spell-like abilities aren't dependent on HD for access. Enter simulacrum, and you've got full access to whatever spells or spell-like abilities you want, so long as something in the multiverse has it as a racial ability. Off the top of my head that includes the full cleric spell list, the full wizard spell list, wish as a SLA, greater teleport at will, astral projection and etherealness, and so much more.