Are they legal? For example could a big company like google go after you if you registered Googel.com As you are clearly trying to profit off of their trademark. I know Googel.com is owned by google so was it converscated off someone who registered it ?
It's trademark infringement if they can show a likelihood of confusion. If you register a domain hoping to get traffic off the goodwill off of a trademark you don't own, you're probably asking for legal trouble. Whether or not the trademark holder will decide to sue you is up to them. There is a difference between generic marks like "apple" and "coined" marks like google. Googel.com was taken from the person who registered it back in 2002. It was a bad faith registration.
Probably not, even though tons of people register these anyway. I would venture that finding a popular misspell for a highly visited single word site is pretty much impossible today.
Apple is not a "coined" term and there are several owners of a trademark on "apple" - each having different usage rights. Registering "appel" isn't going to automatically get you in trouble - but it is going to depend on the usage (including the ads or content) and what your intent is. If you have a fruit basket company called "appel" and use the domain to sell fruit baskets, then you aren't likely to have issues with Apple Corp. If you use it to offer or run ads for Ipods, then that is a different story. Again, trying to make money off the goodwill of another trademark is asking for trouble.
It's called "typosquatting." You can be sued, domain taken, money obtained. It's trademark infringement and designed to capture people looking for a well known brand. Use common sense people...