Open a photo in iPhoto and click on Edit. Now select the Straighten tool. With the mouse cursor on the image and two fingers on the trackpad, rotate left or right to fine-tune the tilt.

Or select the Crop tool and turn on constrain. Now, with the mouse cursor over the image, pinch inwards to shrink the crop frame while retaining the original aspect ratio; move your fingers apart, and the crop frame gets bigger. If you move both fingers around without pinching or spreading, the crop frame will move without changing size. (Unless you run into an edge, in which case it will shrink.) If you turn constrain off, you can move your fingers to change the crop and the aspect ratio, from rectangular to square.

Similarly, you can rest one finger (or, more conveniently, your thumb) in one corner of the trackpad and another finger in the opposite corner; if you move just one, keeping the other in place, the corresponding frame-handle will move. So, with thumb at bottom right and index finger at upper left, move your finger towards the center to pull the upper left corner of the frame inwards; move your thumb to pull up the lower right corner.