Your best bet for “ready to cut” is going to be SVG files, that is the native format that GF converts to, and there are thousands available for free on the internet already. Start looking through the links listed here and download those for use…most of them don’t require any further work, you can just cut them right off the bat.
The DXF format is going to be delayed, but is in the hopper for future development.
JPG, PNG and some PDF files are raster format files. If you collect any of those, you will need to use the Glowforge software (or some other drawing software) to do an autotrace before you can laser them. They will likely require additional cleanup or conversion work before you can use them as well.
AI, DXF, SVG, EPS and some PDF files are 2D vector format files. Those should be laserable without a lot of additional intervention.
Other 2D vector format files that can be converted to the above formats for use are CDR (CorelDraw), and SVG (Inkscape but only the plain version, not the Inkscape SVG)
Programs you can use to convert these to a different format are AI, CorelDraw and Inkscape.
STL, OBJ, 3dm (Rhino) and f3d (Fusion360) are 3D vector format and mesh files. Those have to be broken up into parts and converted into 2D vector format before they can be used in a laser.
Lots of other ones too, but those are probably the most common. 