The US Customs and Immigration Service does encourage e-filing to the extent that it can be done on immigration matters, but attachments, and required notarizations and interviews must follow the initial e-filings.

Every immigration procedure has its own requirements. Usually an e-filing will start the process, then documents will have to be sent in later, which can not be filed electronically.

A completely electronic filing of all documents for important matters such as a green card or citizenship, still is not possible since many in-person activities must be done under the strict requirements.

Yes all of those forms are online! A better question would be, what forms are not online?

My question was about - what forms can be completed and submitted online itself? I know that all the USCIS forms area available for free download on the website.

As of right now you only have a few options for filing immigration forms online. Most new applications must be submitted through mail or in person, because so much other information is needed that you cannot submit online.
Right now the USCIC is accepting online application for replacement green cards (I-90), Travel documents for people with pending cases (I-131), Petition or alien worker (I-40), Application to change/extend non immigrant status ( I-539), Application for employment authorization (I-765), and Application for temporary protective status (I-821).

Basically all the stuff besides the actual immigration applications, and it seems like you would already have some kind of information on file with the USCIC, if you were going to file any of these forms, which may be why they are available.

I have to say the CIC has many more options for e-filing than the United States, but I'm sure you guys will catch up eventually. :P

You can e-file the following forms :

Form I-90, I-131, I-140, I-539, I-765, I-821, I-907