We've mentioned ways get to those ports on the back of your iMac, but if you're using a big PC tower, you can give yourself easy access to the back just by propping it up on an old skateboard.
This won't work if you have your PC in a dedicated cupboard or shelf on your desk, but if you have your PC on the floor, this is a nice trick. Pictured above is my brother's computer, the back of which is a pain to access — you have to literally crawl under the desk and tip the tower over to reach everything (granted, cable management isn't exactly optimal either). After taking the advice of reader Eminator and placing an old skateboard under his tower, though, getting to the back was a snap — all I had to do was roll it out. As long as your cables are long enough, you should be able to reach everything with ease.
The only annoyance I found was that the skateboard was longer than the desk was deep — which is something that will differ from desk to desk and board to board (and may not be an annoyance at all if you've got enough legroom under your desk to begin with). So if you've got an old skateboard or similarly wheeled object lying around, try throwing it under your tower. Note also that, depending on the original rider's preferences, you may need to tighten the trucks (the metal axels on the bottom), otherwise it'll be pretty wobbly and your tower could fall right off if you kick the board.