There is a document that allows you to enter the US whenever you want (contingent on inspection and admission by a US border agent each time)… and it's the US visa. Some can be valid for as long as 10 years and allow you to enter the country many times.
Under US law, length of stay is an entirely separate issue. Unlike what you seem to believe, you would not typically get a “90-day visa”. The visa can have much shorter or much longer validity than the time you will be allowed to stay in the US.
However, you cannot easily get a document that would allow you to stay “however long you want”. That determination will be made at the border but if you do more than one or two long trips in a row, you are going to raise suspicion that you are actually living in the US. For repetitive multi-month stays, I think you would need to seek an immigrant visa and resident status. Same thing if you cross the border many times and end up staying in North America for a long time.
For those of you that don't need a visa for short visits (e.g. citizens from visa-waiver countries), renewing the ESTA every two years is the least of your problem. It's relatively cheap and easy. It's not being denied entry if you are spending so much time in the US without proper resident status you should be concerned about. Note that you would be granted 90 days of stay in the US on the first entry and going to Canada would not usually “reset the clock” so you would need to come back to Europe after three months.
Once again, the length of stay is the most pressing issue, not renewing the visa or authorization nor leaving and entering the country repeatedly.