You can boondock for free on the south side of highway 89, west of the dam. This spot is (as I recall) the first left turn after passing the Visitor Center when heading west. It's about 1/2 mile from the highway, uphill on a gravel road. I got off the road at a small turnaround right at the top of the hill. The view of the rocks is great, and there's even a small view of the lake.
If you go further west, there are additional "dispersed" camping areas along the dirt/gravel roads that provide access to the area on the south side of the highway (the side away from the lake).