Ontario Parks now lets you book backcountry Camping reservations online. Algonquin and Killarney now join Quetico, Kawartha Highlands, The Massasauga and Bon Echo in the ability to check on availability and book a site.
First choose your park, then select 'Backcountry' from the pulldown menu. In the case of Algonquin Backcountry, choose "Algonquin Interior", then "Algonquin Canoeing". The map system will let you reserve a site on a given lake, but not the site itself.
Killarney Hiking Trail & Algonquin Ranger cabins still have to be booked through the call centre.