Does there exist a simple way to measure the download/upload speeds on one's Internet connection? I've always been interested in knowing the extent to which ones "actual mileage may vary..."

BTW, I'm running an iMac w/ OSX...

There's a ton of internet speed test sites available.

Edit: Thanks for updating your question AFTER I posted.

NetSpeedometer (http://www.macupdate.com/info.php/id/20248/netspeedometer) will let you monitor your speed graphically, providing you're running 10.3 or later.

Yes as curlyben said there are lots and lots of sites offering internet speed test I mostly use this site www.ip-details.com (http://www.ip-details.com/internet-speed-test/) because they are offering speed test for unlimited time at free of cost...