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.
If you use PNG and maintain the transparency (rather than using a white background or leaving the canvas layer on), you should get a really clean transition from png to svg, without need to clean anything up, assuming you don’t use an ‘art’ style brush that fades at the edges. I use the too that lets you select a custom shape and then fill it with the paint bucket, rather than using any brushes. Those might not be the actual names of the tools. I just know the little icons and name them in my head
Also, if you have a really detailed design, making it larger, converting it to svg, and then resizing it to the size you want will make a really big difference.
What is difference between Inkscape SVG and regular SVG? Is that why SVG’s sometimes saved in Inkscape and opened in Corel have some wierd 'growth". Like someone grabbed a node or two and wandered away, dragging it with them?
I regularly open Inkscape SVG files in Affinity Designer and have also opened them in Illustrator but they lose their layer names in Illustrator. Here is a Montana map Inkscape Illustrator file opened in Affinity. Note how all the layer names even come thru!!!