find filenames NOT ending in specific extensions on Unix?

You could do something using the grep command:

find . | grep -v '(dll|exe)$'

The -v flag on grep specifically means "find things that don't match this expression."

find . ! \( -name "*.exe" -o -name "*.dll" \)

$ find . -name \*.exe -o -name \*.dll -o -print

The first two -name options have no -print option, so they skipped. Everything else is printed.

Or without ( and the need to escape it:

find . -not -name "*.exe" -not -name "*.dll"

and to also exclude the listing of directories

find . -not -name "*.exe" -not -name "*.dll" -not -type d

or in positive logic ;-)

find . -not -name "*.exe" -not -name "*.dll" -type f