Well we HAVE to start with thingiverse, right? And my chosen frog here is drawn by MorenaP and has been printed multiple times looking really great. No need to sign up on thingiverse, you just browse the huge archive and pick your favorite objects to print.
123D from Autodesk has lots of objects you can download for free after signing up. My favorite frog I’ve seen is this Eastern Banjo Frog, by Autodesk. Files are already in stl format
3D CAD browser has many many cars, but there are also animals, architecture and more… As always, free download in many formats after signing up. Mi frog this time is this one from Johny
Grabcad: no need to sign in to download the files, there are many files but just a portion of them would be 3d-printable.
my frog this time is by Berti Jonsson , but this paper clip also looks fun (by Harold Watkins)
Shapeways: really cool designs but almost nothing is free. You can pay for downloading some of them (and they really are worth it) or just ask them to print them out. No free frogs this time, but I had to show you some that you could buy that look gorgeous, by sigma6289 and Eric Thorsen
3d warehouse: tons of models of all kinds, but you should search with care to find printable ones. I like this Reggorf – the anti frogger cartoon by Rachael but it certainly is not printable with those eyebrows…
3d via: as before, sign up and you can also download free objects. My chosen frog this time is by elander, but it really is a big nice toad, not a frog.