Preparing figures for a scientific article is a time-consuming process. If you are using the LaTeX document preparation system in an environment where you can include PDF figures, then the extensible drawing editor Ipe may be able to help you in the task. Ipe allows you to prepare and edit drawings containing a variety of basic geometry primitives like lines, splines, polygons, circles etc.
Ipe also allows you to add text to your drawings, and unlike most other drawing programs, Ipe treats these text object as LaTeX text. This has the advantage that all usual LaTeX commands can be used within the drawing, which makes the inclusion of mathematical formulae (or even simple labels like "qi") much simpler. Ipe processes your LaTeX source and includes the PDF rendering produced by LaTeX in the figure.
In addition, Ipe offers you some editing functions that can usually only be found in professional drawing programs or cad systems. For instance, it incorporates a context sensitive snapping mechanism, which allows you to draw objects meeting in a point, having parallel edges, objects aligned on intersection points of other objects, rectilinear and c-oriented objects and the like. Whenever one of the snapping modes is enabled, Ipe shows you Fifi, a secondary cursor, which keeps track of the current aligning.
One of the nicest features of Ipe is the fact that it is extensible. You can write your own functions, so-called ipelets. Once registered with Ipe by adding them to your ipelet path, you can use those functions like Ipe's own editing functions. (In fact, some of the functions in the standard Ipe distribution are actually implemented as ipelets.) Ipelets can be written in Lua, an easy-to-learn interpreted language that is embedded into Ipe, or also in C++. Among others, there is an ipelet to compute Voronoi diagrams.
Making a presentation is another task that requires drawing figures. You can use Ipe to prepare presentations in PDF format. Ipe offers many features to make attractive presentations.
Ipe tries to be self-explanatory. There is online help available, and most commands tell you about options, shortcuts, or errors. Nevertheless, it would probably be wise to read at least a few sections of this manual. The chapter on general concepts and the chapter explaining the snapping functions would be a useful read. If you want to use Ipe to prepare presentations, you should also read the chapter on presentations.