I have a problem keeping my camera level when I'm in the field. When I get home I look at my pictures and they are obviously not level. My water looks like it's running out the left or right side of my photo. For what ever reason when I'm looking through the view finder I think I'm level, but in fact I'm far from it. I need some help keeping my photography level.
The best tool I have is a hot shoe bubble level. This inexpensive little device works just like a carpenter's level. There are two or three bubbles inside a plastic cube that attaches to your camera hot shoe. This first one is available from Adorama for only $5.95 with free shipping. You attach it to your hot shoe, put your camera on a tripod, and adjust the camera until the bubble is between the lines. If you have your camera on it's side shooting a portrait orientation, then you turn the bubble to attach on one of the other sides. Pretty straight forward and easy to use.
I also have a triple bubble level, which eliminates the need to reorient the level when switching from landscape to portrait orientation. A little bulkier and $24.95 at Adorama.
I recommend having one of these in your camera bag and using it when ever it's important to have your camera level.
My Canon 7D has a digital level built in. I can turn it on and tell if my camera is level by looking at the LCD display on the back or even in the viewfinder. Having it displayed in the viewfinder is very handy when you're shooting hand held.
Here's a case where having the camera level is essential.