How can I replay Apache access logs back at my servers to do real world load testing?
You can use Jmeter's Access Log Sampler component.
There's a short tutorial on it's use in this PDF.
Assuming all you've got in your access log are GET requests, and you don't mind the requests bunching up at the limit of resolution of the timestamps in the logs, about 10 lines of $SCRIPTING_LANGUAGE should do the trick. POSTs, cookies, HTTP auth, and more subtle timing are a far more interesting exercise.