In combat, a familiar can Help one action every round, if your DM is all about RAW.A stricter DM might apply limits. Particularly regarding what enables sneak attacks, but that's another topic.
Out of combat, sure it can Help the ranger track, but you could do that yourself, without a familiar; does the familiar have abilities which enable it to Help in ways that its owner cannot? - Ah, it might have senses you don't.
The ability to dismiss and re-materialize a familiar seems abusable. For example, you're at a door. Send your familiar to Dimension X, then cause it to re-appear on the other side of the door, and now you can see what's on the other side of the door. Simple fix: only allow re-appearing at an unoccupied spot, within range, *that you can see*.
Can a familiar be dismissed while carrying a (small) item? If so, does the item drop, or go with the familiar? If the latter, then congratulations, you have a nigh undetectable way of smuggling small objects. Even with that, a familiar can steal any item that a rat or raven can reach and carry, so it could be very useful to some Rogues. If you cast Invisibility on the familiar, this widens the options even further; it's almost like Arcane Eye plus Mage Hand. (What other spells might be useful to cast on a familiar?)
Could a sufficiently high-level wizard use Polymorph on their familiar? If so, some forms may have quite useful Actions other than Attack. How about PTP (True Polymorph, made Permanent)?
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