I've been volunteering at the SPCA hospital for 10 years now. I got all my animals there, and I'm currently fostering a really old yorkie for the week until his cruelty investigation finishes. The organization may have some flaws, but the people there are top-notch.
I used to also volunteer at the Vancouver Aquarium and got to do such cool things like feeding the otters and the tropical fishes. It was oh-so-fun, but i just couldn't fit it around my schedule any longer so I had to say good-bye after almost 10 years.