As one of its daily deals, Woot offers the refurbished Dell Optiplex 780 Intel Core 2 Duo 2.9GHz Small Form Factor Desktop for $89.99 plus $5 for shipping. That's the lowest total price we could find for a refurb by $85. It features an Intel Core 2 Duo 2.9GHz dual-core processor, 4GB RAM, 250GB hard drive, DVD burner, Gigabit Ethernet, DisplayPort, and Windows 7 Professional 64-bit.
Also available is the refurbished Dell Optiplex 380 Intel Core 2 Duo 2.9GHz Small Form Factor Desktop for $89.99 plus $5 for shipping. Its specs are mostly as above, although it lacks DisplayPort output. (A full list of differences is available on the product page.) Both deals end today.
A 1-year Advanced Skyline Technology warranty applies to each system.