As Caleb mentioned, a table saw is the easiest way to shave off 1/2", since it will also guarantee that the newly-cut edge is parallel to a flat edge registered against the fence.
A circular saw with its base/shoe registered against a straight board also works, as Caleb mentioned, but can be a pain to set up because of the offset from the shoe to the blade. (I can't tell you the number of times I spent 10 minutes or more measuring and fine-tuning the setup for a cut, clamped everything in place, and forgot to account for the offset. Sometimes I realized it before I made the cut, but not always.)
To eliminate the offset from the equation and save yourself from a lot of wasted time and/or mistakes, consider making a zero-clearance straightedge guide or homemade tracksaw.
A homemade or commercially-available parallel edge guide like the Kreg Rip-Cut would also work reasonably well with a circular saw and will require less setup than a straightedge for repeat cuts.
Another solution is to use a router. You can either use a straightedge guide similar to one of the circular saw guides, or you can use a (top-bearing) pattern bit or (bottom-bearing) flush trim bit. Flush trim bits are slick because you can construct your project with the excess edge extending past the adjoining face, then use the flush trim bit to trim the edge exactly to the face. Whenever you use a router, remember to take light passes.