Well, Thoughtworks have just released a product called Mingle that looks to offer the same flexibility as a "Post-It notes and a wall" solution in electronic form. It's an interesting offering and I'm keen to see it in action. The offering itself is a subscription based pricing, but if your a community or small teams (5 people or less) it's free. Considering that the competition to the software is a pen and paper offering it's a price point that makes sense.
I've just registered for early access and hopefully I'll be able to get a hold of this and compare it to other tools I've looked at previously. But let me reiterate - you should get your agile process right using lo-tech means before investing in any tools. Remember that any tool should be used to support your process; not a means of defining it.