I like the Doggy Steps the best (plastic snap together with a sheepskin covering). The 3 step model is okay for couches (about 12 inches high), but I use the 4 step model for my bed (16" high) and it comes within 5 inches from the top of my bed. All of mine are around 4 pounds they have no problem using them. Of course, you can make your own with just wood and carpeting, but I find these are really heavy and hard to move when you want to vacuum.
They are selling them on Ebay and at Walgreens, Target, Walmart, but the 4 step model is harder to find.
www.doggysteps.com you can buy just the 4th step under accessories and it comes with a new 4 step cover.