I've compiled this list. I don't have experience with most of these, but, if you do, please add more descriptive text to your package. If it does not appear, again, please add it. If you don't have the rep to edit, post it in a comment and @ messgage myself or the last editor. Also, if you feel that a certain element should not be in the list, remove it and leave a note in your edit explaining why it was removed.
The big ones:
#1 TikZ. The standard. As Dima said, it's "powerful, flexible, easy to use, and stunning"
#2 PStricks. Probably the second most used package.
#3 The default packages. More used than the others, but not by reason of being more powerful.
Other graphics packages and tools which create images suitable for inclusion in LaTeX documents include:
- Xy-pic - Best suited to graphs and diagrams, but capabilities for other formats.
- ePiX - Best for mathematical figures, creates PSTricks, tikz, or eepic macros.
- MetaPost - Similar to MetaFont, outputs PostScript files. Used by Knuth. Allows direct inclusion in a LaTeX file via the emp package. MetaPost is now integrated in LuaTeX via the mplib library. Using LuaTeX, you can include your metapost figures directly in the TeX/LaTeX file with the luamplib package, without using any external software.
- MetaFun - An extension to MetaPost.
- LaTeXPiX - Windows GUI, exports PGF LaTeX code
- TPX - Another Windows GUI, more flexible outputs than LaTeXPiX
- Xfig - X-Window drawing tool, saves in its own .fig file, but outputs many formats (Including PS).
- Asymptote - A vector graphics language. Can embed LaTeX within the image. Outputs graphics for your document, not code. It can generate both 2D and 3D figures. 3D figures can be included in a pdf file in the PRC format which allows them to be manipulated when viewed in Adobe Reader.
- Inkscape - A very powerful and well-supported SVG editor.
- Ipe - A powerful vector graphics editor, with several snapping modes that make it especially suitable for variety of technical illustrations. Saves in its own .ipe file format, but outputs pdf and eps for inclusion in TeX documents. Uses LaTeX to typeset text, both labels and larger paragraphs. Supports layers and views, which make it possible to "build" illustrations incrementally in a presentation.
- Sweave - A tool that allows you to include R code directly into your LaTeX file. It does much more than just generate graphics, but it makes inclusion of R generated graphics into LaTeX document very easy.
- KtikZ,QtikZ - A PGF/Tikz real - time compiler for Debian/Ubuntu with KDE (it works with Qt also, and it has a new version for Windows, but I haven't tested it yet). It can speed up the drawing time while at the same time allowing to code directly in Tikz code. It has a template option which allows to define user commands in an easy way as well as a menu with many common (and not so common) TikZ constructs.
- GeoGebra - Award-winning free interactive geometry tool. As such it is also a vector graphics editor and a graph plotting software. Supports exporting to PSTricks, Tikz and Asymptote in addition to more traditional image formats. Available for major desktop and mobile platforms.