Formal:
* Bisous (kisses) - This is the most common way to end a letter or email.
* Affectueusement (affectionately)
* Cordialement (sincerely)
Informal:
* Gros bisous (big kisses)
* Bisous, bisous (kisses, kisses)
* Bisou (kiss)
* Avec amour (with love)
* Amitiés (friendship)
Note: The choice of phrase depends on the context and your relationship with the recipient. "Bisous" is generally appropriate for most situations.