I have been using magicJack on a dry DSL line since October 2008. Besides magicJack blocking some exchanges (due to cost), the service works great.
If you don't want to leave a computer on, the nettalk DUO has the benefit of not needing a computer turned on. You simply plug into your router, wait for a ring back and start making calls. But, it is $69.95 for the first year, then $29.95 per year after that.