I had a tough time deciding to put this in apps or accessories, but ultimately it does have a hardware attachment so I put it here.
I'm pretty excited about signing up for this new service. It's called Square and it's a mobile payment system that uses a credit card swiper to automatically accept payments on the go. Signing up is free and it includes the hardware attachment for free as well! The only costs come from per-transaction fees:
Swiped transaction: 2.75% + $0.15
Keyed-in transaction: 3.5% + $0.15
When you think that Paypal charges 2.9% + $0.30 per transaction, this is actually a financially sound process. It works with Android, iPhone, and iPad (and I guarantee BB and WM soon), so it will be very widespread.
I'm most excited about using this for freelance and other services rendered. I started using an online payment service, but that was awkward especially when the service isn't done online. I then switched to Paypal, but again, there's no way of accepting payments unless I have access to the internet (which I usually do with my Droid, but still). Looks like I'll have a seamless and convenient way of accepting payments and I'll be earning a bit more money that would normally be taken out in Paypal's charges.
Once you download the app, sign up with your personal information and you will have access to the website: https://squareup.com/
They will automatically send you a swiper attachment to the physical address you provide during the signup. Sign up now as they are shipped out on a first come basis. As of writing this, there are fewer than 50 downloads in the Android Market, so hopefully I'll get mine soon. The attachment simply plugs into the headphone jack and is ready to use upon opening the app.
Once I get this in, I'll make a full review! I can't wait.