If you're doing something like setting posts on a slanted surface, you'll likely need to measure the exact angle at which to cut their base so that they stand perpendicular with the ground. But you don't have to break out the protractor; you can easily use a level to determine the angle of the cut.
In this video from DIY YouTuber See Jane Drill, we learn how to measure the angle of the necessary cut with nothing but a decent level. It seems logical to set the post aside the slanted surface and then mark it with a pencil, but as Leah demonstrates, that doesn't guarantee that the post is sitting perfectly level when you mark it.
Rather, she uses a scrap piece of 4x4 and a high quality level. (A cheap, mediocre level might not be that accurate.) She places the 4x4 on the incline and then places the level straight up and down aside the wood until it's perfectly plumb. Then she marks the wood up and down when the level is perpendicular to the ground.
She then uses that line as a reference when making a cut with a miter saw. It's just a piece of scrap wood, but it works as a template for the actual posts she's going to set.
Carpentry Trick: Use a Level to Determine an Angle Without Measuring [See Jane Drill via Digg]