Recalling Longstreet's Assault 147 years ago today—also known as the Pickett-Pettigrew-Trimble Charge, also known as Pickett's Charge—I revisited an old but still worthwhile site, Behind the Stonewall, 360-degree Panoramic Images from Civil War Battlefields. It's "dated" technology, the images are small and the resolution could be better, but it's still unique, pretty thorough, and FREE. Here is the menu of Gettysburg panoramas. Here is the one next to the Copse of Trees, which the Confederates targeted in their long march.
Virtual Gettysburg is a nice site, with better images and a large sampling of some of their tours, but you'll need to purchase the CD-ROM set to enjoy the fruits of what looks like considerable labor. A really nice feature there is the database of battlefield monuments, which you can browse at random, or search for in detail. Looking for that teepee monument? There it is, the 42nd New York Infantry, south of the copse of trees.
Another favorite Gettysburg site is Gettysburg Daily, where you'll find outstanding photographs, and expert captions and essays with a new post every day.
If you're really captivated by Gettysburg and wish to explore the topic with a community of well-read devotees, check out the Gettysburg Discussion Group. Sign up for the regular emails or the digest—the traffic there is steady, and so there's always something new to read in ongoing conversations. Send them a few bucks to help maintain their servers, and take advantage of expertly-led, GDG-sponsored tours.
There are some good resources at the NPS official site as well.