This wiki supplies cartridge ROM images in two formats. They will either be .bin and represent the raw data content of the cartridge or they additionally are available as .crt file for quick emulator use. This may not be possible for some ROM dumps due to no CRT ID being available for them (yet).
Most emulators will support attaching cartridges in .crt format and will then tell you that the cartridge is probably not supported. Attaching .bin files is supported by VICE on a very large scale of files though many are not listed in the GUI. Running VICE on a console or in a dosbox using x64 -help will expose many secrets.The supplied .bin files are ready to be programmed into EPROMs of suiting size when using a modern PC eprommer. A standard C64 eprom burning program will expect the data to be delivered with a start adress at the file's beginning which forces you to manually add the two bytes. Common are $1000, $2000, $3000 and other flat memory addresses which are simply prepended to the file in low/high-byte order. Your eprom burner manual will probably outline the start address topic as well.
When facing an unknown ROM and not being sure which type of .crt may suite there is no other way than testing the available ones. The list of emulated carts is always growing but just game cartridges can be seen as fully supported yet. Most speeders, freezers and other exotics will require their own implementation.
There are several converters for .crt <> .bin which are available for various platforms:
cartconv is installed with VICE and currently covers the most extensive set of cartridges. It's functionality is usually closely aligned to the assigned CRT IDs. cartconv allows conversion in both directions.
mcart by Markus Brenner converts the most common cartridges flawlessly into .crt but does not reverse the process. Download mcart here.