Hell Teacher Nube, although closer to Sealed Inside a Person-Shaped Can then usual. Although he's extremely skilled and powerful even without the help of his Oni Hand, many of the critters he encounters would be unbeatable or at least extremely difficult to approach without it.
The Aburame clan of Naruto act as human hives for armies of destruction bugs, who live off their chakra and can swarm and poison enemies.
In Creature Tech, an alien parasite rips Dr. Ong's heart out, then attaches itself to his chest and serves as a replacement heart. It also provides him with several extra limbs, skills in hand-to-hand combat (the thing can learn kung-fu by watching movies), and when Ong gets really upset, teleportation powers.
In Iron Man, War Machine had at one point Eidolon Warwear.
Marvel Comics' Sasquatch, from Alpha Flight, originally thought he was that way due to gamma rays (a laHulk) but it turned out he opened a doorway into another dimension, through which a Great Beast was able to come through and bond with him.
Toxin from Marvel Comics. Venom tries to be this at times, depending on the host (Eddie Brock pulled of a decent Anti-Hero, Mac Gargan faked it, and Flash Thompson is this until the symbiote freaks out and starts eating things). Anti-Venom tries, but Eddie's a little... off in the head to pull it off for extended periods.
Resident Evil: Apocalypse. The T-Virus gives Alice super abilities, which she uses to fight zombies.
Severed Ties (1992): a desperate scientist injects himself with a reptile based regeneration serum to return his arm count to two, which worked out fairly well as long as one fed it and didn't mind the teeth, or mouth, or eyes.
Parasite Eve. For the most part, the Neo-Mitochondria (Mitochondria who have developed a Hive Mind sentience) are simply The Virus, but in a few people with just the right DNA (read: Aya Brea) it instead becomes a powerful, symbiotic force, granting Super Strength, Hyperactive Metabolism, Pyrokinesis, and the ability to kill people on the cellular level with your MIND. And probably pick out winning lottery-tickets, too.
Dark Future: Krokodil becomes the Host of the Ancient Adversary, the Pawn of the Nullifiers. Exactly what the nature of the Ancient Adversary is is never made entirely clear during the series. However, it seems to take the form of a crocodile and is connected in some way to the Moon. It's certainly been around as long as Seth and the Dark Ones have and it's probably not good, just antithetically opposed to them. Being it's host gives Jessamyn some degree of supernatural invulnerability and some extra super-strength on top of the cybernetic augmentations she already had.
Blending with a symbiote in Stargate SG-1 can result in this: the human (or other creature) gains greatly increased healing abilities, a near-perfect immune system, and an elongated lifespan. They also get the sum total of all the knowledge of the symbiote. The downside? They have to share their body and mind with another intelligent creature. The Tok'ra believe that equal sharing of the body between symbiote and host is the proper way to do this, and only take unwilling hosts in extreme circumstances. The Goa'ulddo not share such morals.
Alex Mercer from Prototype might count, having survived the Blacklight Virus and gotten powers from it. Subverted though, it turns out it's not that he survived but that the virus is him. The guy 'hosting' him was also a complete bastard - so spectacularly evil that the flesh-eating super-virus that ate him was an improvement.
James Heller, same powers, is also an example; perhaps even a purer one.
Shin Megami Tensei: Nocturne has Lucifer inject the main character with a parasitic demon called a magatama near the start of the game, transforming him into a half-demon.
Skullgirls has a couple of these, most prominently Filia, who plays host to Samson (not the one from The Bible). She combines her own fighting prowess with Samson's Combat Tentacles.
Abbey from Gnoph. Her powers come from the symbiote Scut, who lives in one of her lungs. Gnoph in general work this way, and are implied to be in common use on Abbey's homeworld.
In MIB: The Series, when a Symbiote attaches to you the upside is nigh immortality via healing/rejuvenation, rubberman-like expansion tied to an epic shapeshifter baggage, varying amounts of shapeshifting for yourself, on and off again ability to breathe fire, and a "lay on hands"-ish ability to heal others. the downsides, however, are that you could get attached to an annoying Symbiote and you have 20 hours before you mergepermanently.
