I spent a good chunk of this morning fighting with Paint Shop Pro, where I had created an old version of the Invasion map. In honor of that frustrating experience, here is a list of some of my favorite online tools and resources.
nanDECK: This an amazing tool, specialized for laying out cards. It uses its own scripting language, but the scripting is easy and the manual and tutorials are helpful. It can import data from a CSV file, create your cards according to your script, and output a PDF at the touch of a button. A mini-IDE and CSV editor are built right in. As the author’s hobby project, there are some quirks in the behavior and writing, but don’t let that stop you: this is an easy-to-use and powerful tool.
Inkscape: This is a free vector graphics application. It’s not a perfect Illustrator replacement, but it’s powerful, novice-friendly, and I can usually make it do what I need. You often hear this mentioned in the same breath as GIMP, which sells itself as a Photoshop workalike. That’s unfair: Inkscape is a fine program, while GIMP is a morass of bugs, terrible usability, bizarre nonstandard UI choices, and missing features.
Pixlr Editor: I was only recently introduced to this raster graphics tool. It’s Web- and Flash-based, but it’s amazingly full-featured, free, and quickly my new favorite free raster graphics tool. Paint.NET is fine but not quite there (plus you have to install it, so early-21st-century) and you already know my feelings on GIMP.
Game-Icons.net provides hundreds of icons useful in games, with a Creative Commons license that is attractive for prototyping. The icons available lean a bit towards fantasy and fighting. NounProject is a similar resource that contains a much broader array of icons, although they don’t share the unified graphic style of Game-Icons.net.
Incompetech Graph Paper: this bizarrely named site provides very good graph paper generators including hex grids, triangle grids, and dot grids. Gridzzly is a quicker, less full-featured tool that provides the most common papers you might want to print.