Walkers won't have difficulty finding great places for day-hikes and longer treks in Italy. There are few fences keeping you out of places, and most everyone will know of an ancient trail their parents used to go to market, or a trail passing by their favorite forraging spots (these, as you might expect, are described rather vaguely). You'll find trail blazes all over Italy.
There are long distance walking trails in Italy, of course, but there is also a reawakening of the spiritual pathways like that of the Via Francigena, the "Frankish Way" from Canturbury to Rome.
And there are planned treks, like the ones proposed by mapmaker Giovanni Visetti, called MaraTrails. These are free, planned treks you can do at your own pace; the longer segments are carefully planned to be walkable from inn-to-inn. Each trek is mapped and elevations are graphed so that you know exactly what you're getting yourself into. See the Pogerola-Cerreto-Pogerola trek for an example of Giovanni's fine work.
As if that wasn't enough, Viovanni runs the Free Ramblers which offer treks around the Amalfi Coast.
The Valcamonica - Great trekking, great rock art.
- Classic Cinque Terre Coast Walk
- Hiking the Cinque Terre, from Montenero to Volastra and Corniglia
- Hiking the Cinque Terre, from Riomaggiore to La Madonna di Montenero
- Vineland Paradise: Hiking the Italian Riviera's Cinque Terre
A Path to Lunch - Walking trails and trekking in northern Tuscany and southern Liguria.
La Via Della Costa is a new route up the Ligurian coast that ultimately takes you to Santiago di Compostela. 350km in 12 stages.
The province of Firenze (Florence) has set up a great site with trekking maps of the natural protected areas in the region: Aree naturali protette e sentieri in provincia di Firenze (in Italian).
Strada del Vino Colli di Candia e Lunigiana - A walk, mostly along the Via Francigena in the Lunigiana. What I like about it is that it's titled "A walk among marble and lard."
Puglia - The Foggia region has a nice Trail Map and Itinerary (look under "Materiali Promozionali" for links, or select the first one for a pdf with all of them). Also see an interactive map of Puglia.
Wandering in Venice - yes, you can even trek the big cities, looking for treasures overlooked by tourists.
Hiking in the Val Camonica - An account of a winter walk in the Cal Camonica. This valley has the highest concentration of rock carvings in the world. See our suggested hikes with our Val Camonica map.
Giovanni Visetti: Hiking Guide and Mapmaker - a good resource for walks and walking maps, especially along the Amalfi coast and the Island of Capri.
Wandering about in Rome: TRASTEVERE - a good account of wandering through one of Rome's most interesting neighborhoods.
Vagabonding in Salento - Puglia's charm revealed.
Walks on the Wild Side - A writer walks Basilicata, the region around the instep of Italy's "boot".
Walking with St. Francis - Walking around Assisi, in the footsteps of St. Francis.
The Via Francigena in Europe - What you need to know about the pilgrimage route from Rome to Canterbury. Also see, The Via Francigena in "The Valley of the Abbeys" from Initaly.com and our own Map of the Via Francigena.
Alpi Apuane Trials - Itineraries for walking in northern Tuscany, in Italian.
See Tuscany - Urban Trekking - Four urban treks around Siena.
A good source of Italy maps for trekking is TrekTools.