What can I do to make figures I've created generally available so others can use them?
You can 


License them under a Creative Commons license that allows reuse. Put the figures with a notice regarding the license terms on your own website.
Contribute them to Wikimedia Commons. If appropriate, you might also add them to the Wikipedia articles for the methods they are about.
Post them on a site like Figshare which gives them a DOI, to make it easier for others to cite them when they use them.
If you're willing, also make the source code available under a permissive license (with whatever disclaimers you want to put about the code not being so beautiful...) so that others can create modified versions more easily. (You can put code on Figshare, too.)