While writing my comparison of Azure and App Engine pricing yesterday, I had a thought about how to increase some of your quotas without actually paying anything. I'm not sure whether Google would consider this "gaming the system" and stamp on you (and I certainly have no need to do it with my traffic levels), but I thought I'd post it in the interest of sharing.
If you check the App Engine Quotas page, you'll see the differences between the free quota, and the billing enabled quota. For example, if you don't have billing enabled, you may serve 1,300,000 requests per day. If you have billing enabled, you get 43,000,000 requests per day (other limits, such as CPU/bandwidth still apply).
So, if you've taken a look at how the billing works, you'll know that you get to decide exactly how your budget is split between resources. The minimum budget you can set is $1/day.
Consider what would happen if you enabled billing, but assigned the whole budget to something you know you won't exceed (eg., if you don't use memcache, assign it to memcache). Your app will be given the higher "billing enabled" quotas, but it won't cost you a penny!
As I said earlier - I've no idea whether Google will frown upon this behaviour, so if you do it, it's at your own risk!