1) Concept by ex-Air Force folks to build air breathing craft with large JP-4/LOX rocket in the tail. Concept drawing looked very much like an old British Vulcan bomber. Craft would launch from Edwards area and cruise on turbojets to an existing military area off the Washington State coast. It would hit a military tanker loading up on JP-4 (far more than it would normaly be able to get off the ground with). LOX is carried from takeoff. It would aim south, ignite the rocket, and go suborbital, re-entering military airspace off the California coast. At apogee of the flight at perhaps 300k feet and several mach, it would open large doors in the top not unlike the Shuttle, and kick out an upper stage that would continue into orbit. Performance of the vehicle is perhaps only a bit higher than the X-15 was. So it's very do-able. The advantage is in the ability to carry an upper stage internaly, and eject it during the 2-4 minutes it spends in "space".
2) Basically similar idea to No 1, but rather than carry air breathing engines for takoff, would be towed by 747/C5 class aircraft. Idea was tested by towing an F106 with a C141 a few years ago.
Those are two of the ideas that stick in my mind. There are others with exotic scram jet mid stages that should be considered too. I believe that the material science has been figured out, if you use Shuttle style tile thermal protection. These tiles have been bashed in the media, but I believe they would work fine as long as you don't put them next to a large external tank and accelerate the stack to hypersonic speeds exposing them to extreem shock waves in the space between shuttle and tank, and bashing them with FOD.
There are other ideas, but it's too late to think of them right now, and I think the above are the best.
There is Pegasus, with over 100 successful satellite launches, similar idea. They even used that to test the scramjet. While you wouldn't see all the ideas in use all at once, they are in use.