Mine is adapted from the Magic Rating rules in Unearthed Arcana.
Every class has a base Magic Rating (MR) progression. MR from all classes stack. It is used to determine the caster level of all spells cast by the character. Hence, a character with MR of 7 from cleric levels and MR of 8 from wizard levels casts both cleric and wizard spells at a caster level of 15.
The following classes increase MR at the rate of 1/level (as fighter BAB): bard, cleric, druid, sorcerer, wizard.
The following classes increase MR at the rate of 3/4 levels (as cleric BAB): monk, paladin, ranger.
The following classes increase MR at the rate of 1/2 levels (as wizard BAB): barbarian, fighter, rogue.
Hence, a fighter 18/wizard 2 has a MR of 11 and casts wizard spells as a 11th-level spellcaster. However, he only has access to spells and spells per day as a 2nd-level wizard. He gets five missiles with magic missile, but can only cast two such spells per day (plus any bonus slots for high Intelligence).
A 20th-level paladin would cast his spells as a 15th-level spellcaster. A paladin 4/cleric 16 casts both his paladin and cleric spells as a 19th-level spellcaster. However, he is still limited to 1st-level spells as a paladin and 8th-level spells as a cleric.