My main speedlight is the Nikon SB-900. Great, powerful, full featured and relatively priced.
A own a second flash, for portrait photography mostly. The Sigma 530 DG. Super cheap, like below $200 on sale. That price is hard to beat.
Unfortunately, it is its best feature. Not that it is a bad flash. It works, sync wirelessly and all. Just not very easy to use.
This Nissin flash is interesting alternative! Check it out.