Others have already mentioned the mini extension cords and power strips specially designed for wall warts. (I just picked up two
8-outlet Power Sentry strips at Walmart for less than $8 each. These have three outlets on one side with extra space and five on the other with more normal space. They are cheaply constructed and are not surge suppressors, the latter being a feature. :)Before the market developed those targeted solutions I became rather creative with ordinary 3-way taps. These can be used both to elevate one wall wart enough to clear another in the next outlet space and to move one or more wall warts off to the side. This all works best when the outlet strip is in an outlet-up orientation.
Cheap ungrounded 3-way taps come in three basic styles: ones which do not rotate the top tap outlet relative to the plug, ones that rotate it 90 degrees clockwise, and ones that rotate it 90 degrees counter-clockwise. (The distinction between the last two is important when you are dealing with polarized plugs.) It helps to have an assortment of styles on hand. Heavier grounded T-style taps are also useful when (obviously) you have a (less common) grounded wall wart and when you want a slightly more robust mounting and/or side-hanging arrangement.
Dan Lanciani ddl@danlan.*com