Click on it in your inventory. You will automatically transform back after awhile, or you can just click on the statuette again.
Frogs are fast, even if you are encumbered, so are handy for traveling over areas you have already cleared in order to visit traders, or run through groups of opponents, etc.
Your alignments change with your form. The spider statuette can be used to explore orc territory in peace (handy for the two army quests), or chase merchants out of their shops and loot (doesn't help with Kistandalius in Ars Magicana). Orcs are hostile towards cats, though, so will still attack you in that form.
I don't know if the cat or bee/beetle statuettes have any particular uses. When I first found a cat statuette I expected it to be helpful to sneak somewhere or spy on someone, but I never found a need for it. The beetle form may be able to attack shopkeepers or soldiers, similar to the alignment change with the spider.
