Requires Python, Python Imaging Library and Fontforge so it should be cross platform.
This app converts Dwarf Fortress tilesets into TrueType Fonts. The current TTFs are made by with a closed source and slow (on the order of 30 minutes per font) AutoHotKey script for an $80 windows-only app. If the last sentence made you cringe a few times, this is the app for you. (No offense, Elvang.)
Works with BMPs or PNGs, square or rectangle. Has a handful of settings to help with making one-bit fonts (which fbterm desperately needs). Usually takes less than 30 seconds.
Source is GPL and contains some goodies like the unicode mapping for a DF font and a homebrew bitmap-to-svg converter (optimized for pixel art). Though I think the vectorizer could be a lot better, it works fine at the single native resolution.
Usage: df2ttf.py [options] tileset.png/bmp Convert a Dwarf Fortress PNG/BMP tileset to a TrueType Font. See http://kmkeen.com/df2ttf/ for more. Options: -h, --help show this help message and exit -g, --gamma Enable gamma correction. -d, --dither Enable dithering, implies -b. -b, --blackwhite Create 1 bit font.