As for the OP's question:
I e-mail everything. I'm like others--too lazy to find & hook up the USB cable. The emailATfree.kindle.com address is nice, and I send a lot of .doc and .pdf files that way, as well as books I get from Manybooks.net or PG.
Another thing I do is if I'm hitting the road, and I've got a Google map for directions, I "print to PDF" and then just e-mail that to my Kindle, then I've got the directions waiting on my screen for me as I drive. It's been very helpful in a few cases. You can get this function by downloading the
CutePDF driver.
Instapaper is great--I use it frequently.
Did not realize I could e-mail .mp3 files, but may try that as well.
In any case, this has been a wonderful feature; my wife has a K3G/Wifi as well, and we have a similar but separate address for her unit--I can e-mail to one or both just as easily.