2) (Windows XP, 7 or later) PicPick was at first an alternate to print screen for computers that I use running Windows XP because XP does not have the Snipping Tool installed. However it has turned out to be my go to screenshot tool on Windows 7 computers as well. The main reason is that you can set up a keyboard shortcut for taking a screenshot. Under the options menu you can create a hot key (mine is ctrl+shift+z) to bring up the screenshot crosshairs. Another benefit to PicPick is that is also automatically opens your screenshot in its own version of Paint for you to edit it. Their program has many other options available that Paint does not such as pixilation.
3) (Mac OS) Skitch is my alternative to PicPick for the Mac. It does not have as many options as PicPick but it does have a keyboard shortcut for taking a screenshot (⌘+Shift+5) It also opens the screenshot in its own program that you can edit it from.