There are plenty of free camping opportunities in Lake Tahoe. This comes with a giant however.
Nowadays there is only one free campground in the Lake Tahoe Basin.
Luther Pass Campground
The Luther Pass Campground is 15-20 minutes south of South Lake Tahoe, along Highway 89. The campground sits behind the Big Meadow trailhead and is open May to November.
There are 15 sites spread out enough to give you good privacy, all with tables, fire rings, and food storage. The campground also has a vault toilet. Because Luther Pass Campground is free and along the Tahoe Rim Trail, it can fill up quickly, especially on the weekends. Vehicles longer than 20′ not advised.
Tahoe Rim Trail
The Tahoe Rim Trail is the best option for free dispersed camping in the Lake Tahoe Basin. The 165-mile trail follows the crest that defines “the basin”, but there are enough trailheads that you can reach most areas of the trail within a day. This map shows all of the Tahoe Rim Trail trailheads and a few selected campsites. You can camp almost anywhere along the trail; further details can be found on the map page.
There is technically only one Wilderness area in the Lake Tahoe Basin with free dispersed camping. Mt. Rose Wilderness is in North Lake Tahoe and contains the Tahoe Rim Trail.
Granite Chief Wilderness is just outside of “the basin” but is close enough to be considered in Lake Tahoe. It also has free dispersed camping.
According to the Lake Tahoe Basin Management Unit, you may also camp anywhere within the Meiss Backcountry Area and the Mokelumne Wilderness. These areas are just outside of the Lake Tahoe Basin, about 30 minutes south of South Lake Tahoe.