Check out Lycanthromancer's turtle strategy for Core-only Wizards. It's an exercise in a Wizard 20 vs. a Fighter 20. Sovereign glue Gates to Gated Dragons' foreheads? D'oh, you.
Just take advantage of metamagic and Glitterdust and Color Spray, I suppose.
Druid needs Natural Spell. That's all I know.
Addendum: nigh-infinite Explosive Runes,
http://www.d20srd.org/srd/spells/fireSeeds.htm, sell your spellbook, Candle of Invocation loop, deconstruct ladders for ten-foot poles for a profit,
Addendum 2: Abuse bonus spells for a high spellcasting stat RAW (get levels of spells before your class grants them to you) (Venerable Gray Elf),
Addendum 3: Check out that Enchanter's guide (Planar Bind?), (lesser) Geas, Undead army,
Addendum 4: Momentary Alteration Spelltouched feat, Ring of infinite divinely Quickened Wishes (The Wish & The Word's trick), Silent Image to block LoS and other ingenious uses, Prestidigitate (as a friend of mine would rage, shut their eyelids),
Addendum 5:
http://www.d20srd.org/srd/magicItems/WondrousItems.htm#obsidianSteed,
http://www.d20srd.org/srd/spells/simulacrum.htm?, Astral Projection for free items?,
Addendum 6: "Creation" line of spells for platinum?,
Addendum 7: Leadership feat
Addendum 8: I don't understand reply #2's idea.