Try matplotlib - it's a powerful publication-quality Python package. The interface can be basic Matlab-like for the simple things, and a complete Python API allows you to make it as complex as you'd like.

If you are looking to plot data then Matlab (easier to use but $$) and R (harder to use but free) are very flexible and generate an amazing variety of plots.

Dia (easy to use and free) does flowcharts and diagrams very well.

Gimp (moderately easy to use and free) is a good image editor and format converter.

The Slidewrite Plus (http://www.slidewrite.com/) is a very easy to use (not free, but user-friendly) software for graphing (data plotting). It also has curve-fitting, data analysis functions, and drawing functions. Plus, it offers ease of use, flexibility, robustness, and high quality output.