Tarps and Shaped Tarps are significantly lighter and have more floor area than pyramid shelters. e.g. a 2-person MLD Grace Cuben Grace Duo tarp is 8 oz vs the 16 oz for the DuoMid XL. For many backpackers, Tarps and Shaped Tarps are more suited to places with occasional rain (Summer in the Sierras or the desert of Southern Utah) and/or for camping in more sheltered areas (below treeline, behind large rocks, etc.). Although some skilled and adventurous backpackers use them above treeline in the high mountains and other exposed areas that get appreciable wind and precipitation.
True Tarps have the advantage of being simpler, less expensive, lighter and having more pitching options from “close to the ground for storm protection,” to “airy lean-to pitches with great views.” Shaped Tarps tend to be a more storm resistant, but are are designed be pitched only one way—usually lower to the ground with less headroom and views. Note: I do not recommend a solo tarp—for just few ounces more you get almost double the coverage for a 2-person tarp.
Mountain Laurel Designs True Tarps of note are the Grace Duo Tarps in either Silnylon or Cuben Fiber. The 7.8 ounce MLD Cuben Fiber Grace Duo Tarp is a personal favorite and where I think the upgrade to Cuben Fiber makes sense. Don and I shared one on our Wind River High Route.
Hyperlite Mountain Gear Takes a modular approach for Shaped Tarps. The Echo Tarps have an optional Beak (a front end/vestibule add on for the tarp) and an an optional bug mesh an floor insert. Combined all three for a Echo Ultralight Shelter System (an extremely light “2-walled tent.”) But you have the option to take only the components needed for a particular trip.
For Tarps available off-the-shelf from a major retailer, look at the Sea to Summit Escapist Tarp Shelter from REI or a MSR E-Wing 2 Person Shelter. But realistically tarps are not seriously carried by major retailers and you are better off getting a tarp from one of the manufacturers above (or other well regarded cottage manufacturers).