I own a small business which got bigger and few months ago I got incorporated (Canada).

It's time to move onto WWW and start selling online so I'm wondering what options do I have as far as processing credit card payments OTHER than opening own merchant account?

Which companies would allow me to register with them as a corporation?

I already have a merchant account for POS but it's with a small credit union which would charge me a lot for online sales so I'd rather use a third party.

Have you thought about offering one of the many online payment schemes, like Paypal, Google checkout, etc.
They simply integrate into your existing site.

I don't really like PayPal (had issues with them, personally) but I will look into Google checkout.

My main goal is to find a third party (such as Google checkout) allowing corporate registration and without huge fees.

I understand what you are saying there.
Google checkout has an exception of your use of Adwords, their own advertising program.

Another service that is well used is Nochex: Nochex Merchant Accounts: Credit and Debit Card Processing Online

Now they do offer merchant accounts for online activity.

There's others of course: related:www.nochex.com/ - Google Search

It's really a case of doing some digging and seeing what would suit you best.