I have a tif image file, and a .tfw world file (created from OSM data with Mapertive). How can I (using linux command line) convert that to a GeoTIFF file? (i.e. I want to have one file, not 2). I'm sure it's possible with gdal or something.
So you want a geotiff instead of a tiff with a world file (.tfw). This should be the default in GDAL (http://www.gdal.org/frmt_gtiff.html) so:
gdal_translate -of GTiff input.tif ouput.tif
It will default to geotiff.