I believe this to be the easiest approach:
# foo.coffee
class Foo
module.exports = new Foo()
module.exports.Foo = Foo # IMPORTANT, exports the actual class Foo
# foo_spec.coffee
should = require 'should'
{ Foo } = require 'foo' # Requires said class Foo
foo = new Foo
foo.should.be.an.instanceOf(Foo)