Pack covers. Not highly effective if depending on one to be absolutely capable of keeping the interior of your backpack dry. They can most definitely help, but shouldn't be counted on.
A better strategy is assess the backpack to determine if it can be made reliably water resistant. Start by assessing the flap or zipper closure of the main compartment. Does the flap easily cover by a good margin the opening into the bag? Are the zippers covered by a flap of material? Is the backpack already treated for water repellency or is it mad from a coated fabric? If these are in place, the backpack is already pretty impervious to rain.
If the fabric is not treated (or even if it is), or does not have a interior coating, then you can remedy this.
First, use
a seam seal to seal the interior seams on the backpack. Even a backpack that is already water repellant you want to check the seams inside the pack.
Next, use a
DWR (Durable Water Repellant) treatment to treat the fabric of the backpack, which is easily done at home.
[The blue text are clickable suggestions of products that have good track records, but there are several reliable products that can do the job.]
Doing this will also help keep the fabric of the backpack from soaking in water and increasing its weight.
Waterproof bag liners have already been mentioned, which are big, capacious sacks that sit inside the main bag and come in different flavors, from
compactor bags to
ready made products designed as accessories