NudgeMouser 2 has a mouse lock feature and allows you to click the mouse from the keyboard.
Why is this good, and why is it better than Apple's free Easy Access? NudgeMouser can move as little as 1/6 pixel per keypress, allowing very fine adjustment. It lets you operate the mouse without touching it, thus eliminating mouse jitter and those annoying "twitches" when you release the button. It also allows you to drag things you can't normally control with the arrow keys, such as controls and individual handles in graphics programs. This is a mouse enhancement, rather than a replacement - like Easy Access. It's always on, and it's specifically designed not to interfere with normal mouse usage.