When it comes to sample packs, many people I talk to (non-musicians and musicians alike) don’t seem to be familiar with the incredible amount of power that programs like Ableton Live, FL Studio, and Sony Acid Pro, as well as hardware like the Elektron Octatrack have gained with regard to manipulation of samples and loops. This is understandable because since it’s inception the world of sampling has primarily been a static one. Playback of samples used to only mean reproducing the original source sound as closely as possible and with very little variation. This resulted in repetitiveness and a machine-like feel. But after working with any of the newer tools for a bit it becomes apparent that audio files like samples and loops are now totally elastic. This is not only in the critical playback attributes of pitch and tempo but also any number of other means of warping the original sound into something entirely new and unrecognizably fantastic. So sound packs (also known as sample libraries) are gaining momentum as not only pools of professionally prepared source material that can trigger inspiration as-is for new songs, but also as raw starting points for entirely new re-imagined instruments or tracks, using samples/loops like sculpting clay as fodder for playfulness. With some practice this can not only be accomplished in the studio but on stage!
For more programs designed to be used specifically with loops, see this Loop Tool Comparison Chart.