There's a fine line between utilizable and costing to much for maintenance...but if you can keep the distinction in mind you'll be ok.
1. Parse out your network functions and deploy them on paired servers...two old machines may equal the performance of one new one while providing reduncancy. This works well for external DNS servers, internal DHCP machines, print queue managers and the like.
2. Move the old hardware to the lab - and then take the old lab machines over to your nearest school or charity. The non-profit can use the equipment, and your company will get a reasonable write off come tax time. Be sure to use an OS that is legal, and wipe the data before your begin.
3. Take the old hardware and use it to host forums for the linux/windows flame wars. Never seem to have enough bandwidth for all the heat generated by that topic.
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