Lion introduced a new feature called "elastic scrolling", in which you can scroll past the top or bottom of the window and it'll "bounce back" like a rubber band. If you find this feature distracting or annoying, you can turn it off with a Terminal command.
Just open a terminal and type:
defaults write -g NSScrollViewRubberbanding -int 0
Then, restart any elastic-enabled apps that are currently running. You should find that scrolling has returned to its old, more traditional method. To restart the Finder, just type this command in a Terminal:
Of course, you can log out or restart your machine to make the changes as well.
If you want to revert back to the default elastic scrolling, just run this command and restart your apps again:
defaults delete -g NSScrollViewRubberbanding
Check out the video above to see the tweak in action. Note that while this should work in many apps, it won't necessarily work in all of them -- it'll definitely work with all apps that come bundled with OS X, though.
Remove Rubber Band Scrolling in Most Apps [MacOSXHints]