I'm not really a fan of having a thousand sub-classes of magic-user, each with its own spell lists, but I do like to have a few flavorful options. Here are a few fairly simple ones I've come up with by playing with the rules for memorization and casting. Only one doesn't use the standard spell lists.
Mage: This is the archetypal magic-user who learns to cast spells through rigorous study. The mage must memorize spells, as per the standard rules, but can memorize extra spells per level equal to his or her Intelligence adjustment (minimum of one spell per level that the character can cast, in the case of a magic-user with an Int penalty.) For example, a 3rd level magic-user, according to the B/X rules, can memorize two 1st level spells and one 2nd level spell. With Intelligence 16, a +2 bonus, she can memorize four 1st level and three 2nd level spells. If her Intelligence were 8, she could memorize only one of each.
The mage can actually cast only the number of spells per level per day given in the table, unmodified by Intelligence, so our 3rd level mage can cast two 1st level spells and one 2nd level spell, regardless of her Intelligence. Any memorized spell can be cast as often as desired within that limit, however. For example, if the mage has Sleep, Read Languages, Protection From Evil, and Detect Magic memorized, she can cast any one of them twice. It's not fire and forget; it's rehearse and cast as needed. The magic-user with higher Intelligence can remember more spells at once, providing greater versatility, without increasing absolute spell-casting power.
Alchemist: The alchemist casts spells by mixing magical and chemical substances. Unlike the standard mage, who can choose freely amongst her memorized spells when casting, the alchemist must prepare specific spells in advance. He may prepare as many spells per level per day as the spell tables prescribe, plus his Intelligence bonus, and may "cast" them all. If a 3rd level alchemist with Intelligence 16 prepares two Sleep spells, Charm Person, and Detect Magic, then he can cast Sleep twice and the other spells once each. Each spell so prepared costs 1 sp (or whatever base coin the campaign uses) per spell level. His concoctions have a very short shelf-life, remaining viable for only one day, after which they become inert, and new ones must be prepared.
An alchemist may carry a supply of extra reagents to prepare spells in the field; each 100 sp worth of reagents weighs 1 pound.
Spells cast by an alchemist cannot be foiled by magical silence.
Fey-touched: These magic-users are a breed apart, either tainted with fairy blood, or changelings raised among fairies before returning to human society. Fey-touched magic-users are innate spell casters, being able to cast any spell in their repertoire, within the limits of the spells per level per day tables. However, they do not gain bonuses from Intelligence, nor are they able to add new spells to their repertoires by study. Instead, they begin play knowing two 1st level spells, and gain a new spell at each level of experience, determined randomly from the spell levels the character is currently able to cast. Reroll duplicates.
Due to their fey heritage, fey-touched magic-users gain the same immunity to paralysis from the attacks of ghouls that Elves enjoy, as well as the Elven ability to detect secret doors.
Because they need not study to learn spells, fey-touched characters may choose to learn the arts of battle as well, gaining the ability to use any armor or weapon and a d6 Hit Die, and advancing on the Elf experience table. Such magic-users are limited to 10th level, as an Elf. (Elves can be considered to be fey-touched spellcasters by default.) Of course, a fey-touched magic-user can also forego this additional training and focus solely on magic, using the standard magic-user tables without limit, but they still remain curiously unable to learn magic by study as ordinary magic-users do.
White Mage: Taking a more spiritual path than the standard mage, white witches and wizards are able to learn and cast spells from both the magic-user and cleric spell lists (and druidic spells as well, if those are available in the game.) Using any spell which causes direct bodily harm, however, will rebound on the caster, causing equal harm; thus such spells are rarely learned or used. White mages memorize extra spells as standard mages, using either their Intelligence or Wisdom bonus, whichever is lower.