Did a Google search for: practice integrals interesting.

Relevant finds:

[PDF] Practice Integration Problems MATH 182: Fall 2006

[thread on another forum] How Good Am I? - a lot of back and forth between people trying to find a good method, may or may not be what you're looking for

Other possible searches:

practice integration problems

list difficult integrals

practice integration parts

practice integration partial fractions

etc.