Much as a low sofa may seem like an insurmountable problem, it actually is an easy thing to fix. Depending on the style of the sofa, the height you want, and your particular skill set, one or more solutions is sure to work for you -- as long as the arm height and depth of seat remain relatively unaltered and comfortable.
For a quick and easy solution where a slightly altered sofa look is not a style problem, add a foam pad beneath the existing sofa cushions. Purchase an extra-firm cushion, cut to the dimensions of your sofa. Keep the height of the cushion as low as possible while still a comfortable sitting height so it is not conspicuous beneath the regular cushions, and won't change the overall proportions too much. Sew a cover for it from a complementary fabric and use any extra fabric to make a few throw pillows so the sofa doesn't look mismatched.
A Basic Solution
Those with basic carpentry skills may choose to construct a platform. But if building with wood is not your forte, make a rectangular platform using concrete blocks or low pavers laid in the shape and size of the sofa's base. Platforms are a particularly good choice for heightening expensive or vintage sofas whose value would be diminished by alterations. Painted black to make the platform less visible, colored to match the wood of your furniture or covered in matching carpeting, a base of blocks is inexpensive, sturdy and inconspicuous.
The Old Switcheroo
If your sofa has removable legs -- usually screwed into a metal plate on each corner -- the best solution is to remove them and replace them with taller legs. A visit to the hardware store turns up a wide variety of furniture legs. Take one of the original sofa legs with you so you can use it as a guide to choose a taller version of a similar style, and to ensure you correctly match the size of the installation screws.
Out-of-the Box Fixes
An option for sofas with chunky but less visible legs is the addition of a screw-in ball (also called a bun) or other turning designed for use as a furniture leg -- on the bottom of each existing leg. This is an easy and inexpensive fix that is easily reversible should you decide to convert the sofa to its original height. Large furniture casters or specialty furniture risers are other solutions for subtly increasing height.