For Core 2 Duo: Foxconn P9657AA
The P9657AA is a Socket 775 motherboard for Intel platforms that targets the mainstream. At $99 it should be affordable for most users, yet it comes with decent features and will accept every Intel processor on the market today. There are two different models, the 8KS2H and the 8EKRS2H, the latter supporting additional RAID 5 SATA and two Firewire/1394a ports. Apart from that, the offerings are alike.
All three FSB speeds (533/800/1066) are supported, and you may install DDR2-667 or DDR2-800 memory modules of to 2 GB each. One of the SATA ports is available as an eSATA connector, which allows you to connect external hard drives easily. For add-in hardware there is 7.1-channel audio, x16 PCI Express for graphics, two additional PCI Express ports, and three 32-bit PCI slots for legacy cards.
This certainly isn't an enthusiast product, but it is a very robust and mature platform for those who intend to upgrade to an Intel system (preferably using Core 2 Duo).