For me picasa was the first software to offer a seriously easy to use straightening tool. There were some tools with stretch out tilts but most only offered slides and or the ability to type in an angular change. They were not easy to use and generally didn’t show you interactively how the photo was being rotated. Picasa even in its early versions (pre-google ownership) included a straighten tool in the first basic enhance panel. By comparison the picasa straightener put a detailed grid over your image and as you move the straightening slider. Moving the slider to the right is a clockwise tilt and moving the slider to the left is anti-clockwise tilt. The big improvement was that as you moved the slider the whole photo was moved on the screen so it was relatively easy to line up the horizontal or vertical straight lines in your photo.
The on-line end of picasa is now google+ photo (I’m expecting that name to change name soon) and it also has some good basic editing tools. Specifically a decent straightening tool. However it is somewhat hidden because the tile that activates it is called crop, but it does have a curved arrow above the typical crop icon. To find it firstly click on the edit item on the tool bar above your image, which appears above any single photo display. This the brings up a side panel which has a number of tiles representing various tools and filters. At the top of this panel are basic adjustments and these are all green tiles with “identifying” icons. The crop & straightening tile is on the lower right.
The crop/straighten panel (shown above) is then displayed and it has a number of tools to rotate the image. I’m using the ruler called straighten. Using it you first select one point, say on the horizon, click and then you stretch out desired the desired new horizon, and click on a second point on the horizon. The photo is then adjusted on screen and you set where the automatic cropping is also displayed (outside this the triangular salvages are still displayed but subdued). The two icons with the small rectangle and curved rotation arrows can be used to rotate the photo through 90° to right or left (ie correct for the camera held at 90°). The sliders also rotates the photo but only by a limited amount. Like the original picasa straightener moving to the right is a clockwise rotation and moving the slider to the left is anti-clockwise tilt.