this may not be news to a lot of you - but Wavesurgeon is a fantastic program for chopping and editing beats. and it even calculates the BPM - all you have to do is load a loop then it breaks it down into individual hits
Copy-Past-Edit-Silence-Reverse - mangle it all you want
its great for fine tuning any sample
http://www.wavesurgeon.com/download.html
there is also a "links" section for a lot of sample and break related sites