Yeah most image editors that aspire to be half useful will have such a function buried somewhere.
Sticking with the free stuff for now
https://www.gimp.org/ definitely has an option/filter called anti alias, and also has ways for you to construct even better versions from the other filters if you think through it all (do an edge detect and use that as a mask for a slight blur set to a lower opacity sort of thing) or simply replicate existing anti aliasing techniques your graphics card might use (mip smoothing/mip mapping is just resizing up and down very quickly using basically anything other than nearest neighbour scaling)..
http://www.getpaint.net/index.html should have something, some seem to favour a plugin for it. It is far simpler than GIMP and things like it but still very powerful.
Another nice tool
http://www.photofiltre-studio.com/pf7-en.htm
Even something really simple like
http://www.irfanview.com/ can do stuff
http://www.firefromthesky.org/art/iview/trix.html
Several of these programs are options in the installer at
http://ninite.com if you would rather not mess around with downloading and installing them.