It only costs $5.00 a month for unlimited projects (~$0.17 a day) so maybe it's worth a trial for a month or two if you think it's a good fit?
I don't know of any others that were designed from the ground up for GTD although many apps claim that they are compatible. You will have to try them out to see what works for you.
I'm using Appigo's Todo Pro. You can get the iPhone, iPad & Mac versions for free. If you want to sync across devices or use the web-based version then you need to subscribe but it's inexpensive... $2.00/month or $20.00/year (and a 14-day free trial). It's more basic than NirvanaHQ but that's what I want. I've found that the more complex the system is, the more time I spend driving it rather than getting things done.