Imbue with Spell Ability
Spell-Linked Familiar might be a straightforward method, but, truth be told, it's not the first thing that springs to one's mind when talking about how to give a subject that doesn't have spells or doesn't belong to a spellcasting class access to spellcasting: Imbue with Spell Ability is a core spell that transfers a couple of spells to a creature and it does the job perfectly if you want a spellcasting familiar. However, the biggest downside is that Imbue with Spell Ability is a cleric spell, so it won't be easy for an arcane spellcaster to be able to utilize this method by himself: the only feasible way would be in a Mystic Theurge-type of build, but if you've got a cleric in your party or a cleric cohort, by all means convince him to imbue your familiar. This method is really good for Mystic Theurge-type characters since they'll most likely have more spells than what they'll be able to cast in a certain encounter; sure they spend a fourth level in the process, but the cost of quickening three low level spells is more than that. Imbue with Spell Ability is also featured in the Magic cleric domain and while you can use the Arcane Disciple feat to get it to your spell list, it's not a good idea at all, because you can only transfer spells that are cleric spells of specific schools. At its core, Imbue with Spell Ability has a lot in common with Spell-Linked Familiar: both transfer a set of spells within some restriction, up to second level; however Imbue with Spell Ability scales a lot better, since you can transfer the maximum number of spells when your familiar (or you) reaches 5 Hit Dice. This and the fact that the transfered spells are casted according to the caster's and not the recipient's level, makes it a lot better than its alternative. The spells that re useful when paired with Imbue with Spell Ability are, again, mainly low-level buffs: Protection from X, Resist Energy, Resurgence, Benediction, Body Ward, Blessed Aim, Comprehend Languages, Detect X, Divine Insight, Find Traps, Omen of Peril, Vision of Glory, Wieldskill, Close Wounds, Cure Light Wounds, Remove Paralysis and Lesser Restoration.
This method is for: characters that have levels in the cleric class and/or that have a cleric that can cast Imbue with Spell Ability, clerics that have the spells to spare.