Just in case you play D&D in several temporal dimensions at the same time, and managed to play out all of the existing familiar options, Wizards has seen fit to push out another article chock full of the Tiny bastards.
Its primarily geared towards Eberron, but you can use them anywhere with a few name changes when it concerns stuff like dragonshards/marks. The author is quick to point out that magical tattoos, which can be found in Adventurer's Vault 2, work great for replacing dragonmarks...not that we actually know what those will do until next month.
The article has a few quips from future releases that it "recommends", its just too bad that we have no fucking clue what they are/do. As an avid Eberron player, this doesnt effect me at all. Hell, it wouldnt even matter if I wanted to use it in my homebrew games, since I'm perfectly fine to change a few names or fiddle with the prereqs.
Concerning the actual familiars, there are quite a few: 14 Heroic, 7 Paragon, and 2 Epic, and there arent any reprints. Some of them have leader-like qualities, such as the dragonmark reflection which buffs your healing infusion when active, or the clockwork scorpion which lets you or an adjacent ally benefit from its stored poison.
Best Heroic-tier familiar goes to the tome caddy, which lets you use a tome as an implement even if you cant, and when active lets you use a tome hands-free in addition to boosting summoned monsters.
Best Paragon-tier goes to the minor foulspawn, which looks like, "an inside-out cat with three eyes and two mouths." What can I say? I like cats. That aside, it gives you a bonus to Dungeoneering and fear effects, but also can babble a bunch of insane shit at something you hit with an arcane power, dealing bonus psychic damage.
There are only two Epic-tier familiars, so I'll just give best familiar to the lingering nightmare by virtue that the other one is thematic to the Silver Flame, and I just dont care for them. A lingering nightmare is psychic residue that looks like a small cloud that changes with the master's mood. It gives you a bonus to Intimidate and resist psychic when passive. When active it lumps on a random condition whenever you hit something with a fear effect. Each time. Very cool.
There are seven feats, each of which obviously boosts your familiar in some fashion. You can make it bigger, allowing it to flank for a bit, let it move 20 squares farther away than normal, or create an energy shield that reduces damage but also shields your familiar completely from harm. Ranges from nifty to meh, but mostly good.
There's also a set of four magic items, three of which are for the Familiar item slot. One of them is a magical jar that your familiar can 'port into when destroyed. The others are basically various collars that let you teleport to your familiar, grant a bonus to defenses, and provide cover when your familiar is in the same area as a larger creature.
The paragon path, familiar bloodsmith, is an artificer paragon path that lets you regain bonus hit points when you burn a healing surge and your familiar is in passive mode, originate attacks from your familiar when you spend an action point, and get two familiars at level 16 (one be Heroic tier).
Bloodbond rebuke lets you knock an enemy back if they use an attack that has your familiar as a target, rescue familiar lets you teleport your familiar to your square in passive mode immediately after slain, and bloodbond swarm lets you turn your familiar into a mobile zone that deals automatic damage with a slow effect (your familiar is restored when the power ends).