With what I've learned so far, I'd say there's at least 3 big different kinds of free online virtual pets. By different kinds I'm not talking about virtual Cats compare to virtual Dogs or virtual Horses compare to virtual Camels... I'm talking about what you can do or even what you have to do with you virtual pet.
The most basic virtual pets ask you very few attention, and they don't request anything important from you. A perfect example of this kind of virtual pet is Bloggy, my virtual blue dog. Bloggy is always happy, even when I stay several days without visiting him or even if I don't feed him... Bloggy is there just for fun! No more, no less... It's very easy, but it's also very limited... there's not much to do with him...
At the next level of difficulty, you have "simulation games" where you have to do everything like in the real life... meaning you have to feed your pets every day, otherwise they're dying... you have to train them, you have to go to the vet. With some vitrual pets games you can train them to run then you do races to earn money... and you'll need a lot of money to care for your horses! So, those ones are quite demanding... and you'd better be ready to spend some time every day, if you plan to succeed with those.
Then the "last" level is complete virtual worlds like for instance Neopets. With those kind of virtual pets, the pet has not much to do anymore with real pets... Actually, you can explore a wide virtual world where you can buy stuff for you pet... There's lot of things you can do with your pet but you can also meet and chat with other pets owners... You can earn virtual money to customize your pets, etc... So, it's maybe not as difficult as "simulation games" from previous level, because it's mainly for fun, but it's for sure as much time consuming... or even worst!
So, now you have a bit more information about the different kinds of virtual pets. Depending on what you're looking for and the time you're ready to spend to learn and/or to explore new things or worlds you'll know which one to choose...
I'll probably talk back about this and give you some examples for each categories of free virtual pets, but I told you this post would be quick... so... :-)
P.S.: If one of you has a Coolcamel and want it to get married... maybe we could find an appointement! :-))