Spanish design firm OITO designs the M-Table, a transformer which converts with a lift from a coffee table to a dining table, using a base with both short and long legs. It's a clever design without a lot of moving parts;
You pick up one end and pivot the leg to the long position and then go around and pick up the other end, and voila- a stable table.
They have solved a lot of the problems that I faced designing a similar table with Julia West Home a few years back that worked on sort of the same basis:
Our version was more complicated; here you can see it in the small coffee table mode, with the L shaped legs. Note how ours line up in the same direction, where OITO's table has the legs cross through each other.
Our tabletop unfolded to double width and then slid to the center, complicating construction.
To pull up, you would put your foot against the right L base and pull the whole table up; then you had to latch the base to the top to keep it stable. OITO solves this problem by having the sloping leg which makes it stable without a latch.
Then you would unfold and slide the top. The sliding top, the latching mechanisms, the weight all conspired to make it too complicated and difficult to get past the prototype stage. Because the legs were vertical, if you didn't get the latches done up properly there was a danger of collapse.
I find looking at the two is fascinating; same basic concept but two different levels of complexity and sophistication. OITO eliminates the sliding top, making it much simpler. It's therefore possible to make it wider, making it more stable. Then they put the legs on the angle, so that latches are not needed, making it safer.
Really, making something simple is the hardest thing to do.
More images in Design Milk.