Logitech came up with mouse wheel that spins freely.
Its wheel has 2 modes. Notched vs Free-spin. There's a button below the wheel that lets you toggle the mode.
In spin mode, it will keep spinning. (if you push it hard, it can maintain spinning for 28 seconds!) This is very useful for scrolling long pages.
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Most models of their gaming mouse also has flywheel feature. See: Logitech Gaming Mouses Review.
If you are buying them, i recommend their gaming model, because, for the same price, the gaming models are often more feature rich, because the market customers there are more tech savvy.
Spinning is extremely useful when you don't have accelerated scroll.
If you have accelerated scroll, you can scroll a long page by just a few fast flicks. And you can comfortably control the rate to stop at any position of the page.
If you don't have accelerated scroll. You have to push the wheel some 30 times, or move hand to keyboard and use ⇞ Page △ ⇟ Page ▽ keys. Using page keys jump by screenful, then you have to use arrow keys to position it, or back to scroll wheel.
(long pages are common. Most social network sites, g+, Facebook, twitter, by default has long pages, and when you hit bottom it expands, aka infinite scroll.)
So, if you are on Linux, then the free-spin wheel becomes very useful.
On Linux, there's no system-wide mouse wheel acceleration. It's app-dependent, and very few apps implements that.
Worse, in Firefox or Google Chrome, there's no “autoscroll” feature. Without accelerated scroll, you can work around with autoscroll. That is, press middle button, then move mouse to pan-scroll. The rate is dependent on your mouse position. You can turn this on in Firefox, but not in Google Chrome.
See: Firefox: Set Mouse Wheel Scroll Speed, Backspace for Previous Page, Autoscroll
Logitech Gaming Mouses Review