Even if you don't use things like UMD, the familiar grants you Alertness when it's nearby and can make its own Spot and Listen checks (a little like the Quick Reconnoiter feat or similar), as well as the familiar bonus which is equivalent to a Skill Focus, Lightning Reflexes or Great Fortitude. None of those things are all that brilliant on their own (like three feats, but three bad feats) but added together they're not a bad package, especially if you get them for one feat (Obtain Familiar).
There is also Imbue Familiar with Spell Ability from the Spell Compendium which allows great action economy all by itself, plus combos like Mystic Surge (PHB2) where your familiar can use something you couldn't use on yourself (Harmonic Chorus is a great one for Bards). If you Imbue concentration-based spells like Telekinesis you don't need to transfer many slots. Speaking of which, there's the Familiar Concentration feat knocking about somewhere IIRC too.
Share Spells is awesome. Something like Shapechange is bad enough, but casting it on (or Sharing it with) your familiar is crazy. Who said familiars couldn't fight? Share Spells also lets you ignore Personal and type restrictions. It's probably more useful for Animal Companions, but you can do things like cast Enlarge Person on your familiar even though they're not a "Person".
That would be it. The source that gave me the combo didn't list the feat name. But yeah if you absolutely want to use a familiar and plan to do anything except straight wizard 20 that combo is pretty much necessary.
Originally Posted by Cyber_Goddess
Actually, not so much. Your (effective) Wizard levels only really boosts special abilities, Int and NA. BAB, HP, saves and skill ranks are all based off the master's which in turn are based off character level. Share Spells is available at L1. Having a familiar as a Wizard 20 is only a bit better than as a Wizard 1, and the difference isn't so big as compared with having a familiar in the first place. (In Epic, it'd be worth taking Obtain Familiar just to get the higher level because you get Familiar Spell which is quite nice, but that's a bit niche)