One of the most effective multiclass combinations is the rogue matched with sorcerer or wizard, because the synergies build so effectively.
To begin with, familiars. Rogues get great skills, and a familiar can use all the master's skills. If you choose your skills carefully, the familiar becomes a secondary character, rather than a save booster that sits in your pocket. But so many skills depend on being able to talk -- so take a raven as your familiar, and open up the possibility of Diplomacy, Intimidate, Bluff, and more. Sure, your familiar, with its poor charisma, won't be very good at these skills at low levels, but all it needs is a DC10 check to add +2 to your roll via Aid Another!
But the skill that tops all others is Use Magic Device! With that, your familiar can be activating wands and scrolls every round of combat. In essence, your character (that is, your mage/rogue plus familiar) can supplement a normal round with activating an item! It's like having a quickened spell each round. If your DM balks at a raven manipulating a scroll or wand, consider using Unseen Servants to help the familiar, or share an Alter Self to put the familiar in monkey form. The raven's speech, a supernatural ability, is not lost with the change of shape.
There are other nifty synergies to be had. You can cast on your familiar the same spells you might cast on yourself. Buffing up your familiar with Mage Armor, Shield, and especially Enlarge Person gives you an ideal partner for flanking. Let your familiar fly while tumbling on the other side of your foe. Not only does the enlarged raven give you +2 a flanking bonus and a +2 Aid Another bonus on attacks, but while flanking, all your attacks are sneak attacks! Sweet!
If you prefer not to put your familiar in harm's way, get a Rust Bag of Tricks, and let the familiar use it to summon a fighting animal. Put the animal on one side of your foe, and tumble onto the other side, using the animal as a flanking partner. That frees up your familiar to read scrolls.
Then there is always a fun trick for the halfling or gnome. Cast enlarge person on your familiar (from tiny to small), cast reduce person on your self (from small to tiny), and take a ride on your flying mount! It only lasts a short while, but it is still the only way I know for a first level character to actually fly.