Lava Jumps

Lava jumps are probably the most popular and used obstacle in an obby. The obstacle is a number of lava blocks that are wide from wall to wall. Touching the lava is deadly for your character, so it must be jumped over. Some make you die immediately after you touch it, and some casually take a while to make you die, which is very useful. 
Another obstacle made out of lava is checkerboards. These lava blocks arranged in a checkerboard pattern are usually too long to easily jump over in one direction, so they should be diagonally jumped over.
There are also Spinning Lava Jumps, which spin and are much more difficult to avoid.