For me, I found my ideal commuter a short time ago. I used to ride my hardtail MTB with slick tires but invested in an old Mercier road bike I found in a 'vide grenier' (fleamarket) a few weeks ago. I paid EUR 50 for it and it beats my fancy MTB for my 6+ Km commute. It has holes for fenders and racks but luckily, I work for a company that has bike parks and shower rooms, so I only carry a change of clothes in a small backpack and shower before going to my desk.

I have probably spent over EUR 100 on it, but still it's a very inexpensive option. I intend to keep upgrading it, but want to keep it especially because it's not a 'desireable' bike, so I feel comfortable riding it around town.