Oh No! More Lemmings was the first sequel to Lemmings. It was both released as an expansion and a standalone game in 1991 for the Amiga, Atari ST, DOS and other platforms.
As you can imagine from the title, it's exactly the same game; it just offers you... well, more Lemmings! There are 100 new levels, with an excellent difficulty progression. The first levels are easy and the last ones almost impossible.
Some people consider "Oh No!" even better than the original game. Especially if you didn't like Lemmings 2: The Tribes because of the changes it introduced to the formula, you have to try this one.
Download for PC 
Download for Mac