I've had good success in the past with MSI (Micro Star International), Asus, and Abit.  The systems I've built with these boards have generally been very stable and have performed well.  The Abit boards, if they're a newer model that hasn't been on the market long, often seem to need a BIOS update or two before they get completely stable (but once they have the updates they are quite stable).

I've not had good success with anything from Soyo.  I've bought at least 3 Soyo motherboards in the past, and none of them has been stable for me.  As with any recommendation, your mileage may vary.  I've bought both low-end and high-end Soyo boards and all have had stability problems that BIOS updates and so forth wouldn't fix.  I've ended up selling every Soyo board I've ever bought within a few days on eBay (with lots of warnings to the buyer).

To be honest, though, the importance of the motherboard in the system's stability is slightly less critical than that of the RAM installed on the board.  If you run "cheap, generic" RAM on a high-end board, that board will seem unstable.  If you run high-quality RAM on a cheaper board, that board will also tend to be more stable.  I've had good results with Corsair RAM (especially their XMS line, which is pricey but worth it), Micron, and Kingston.