iOS: Priority #1 for any mobile expense tracker should be speed, and you'd be hard-pressed to find a faster app than Next.
When you launch Next, you're presented with a grid of icons representing different types of expenses. Just tap the one you want, add an amount, and you're done. As you spend more in a certain category, its icon gradually turns blue, giving you a one-glance overview of where your money has gone.
Swiping to the left from the main screen shows you an overview of your expenses for the month, and lets you delete individual entries with a swipe. Sliding to the right from the home screen reveals a statistical breakdown of your expenses, complete with beautiful graphs. It might not have as much information as you'll find in some other budget-tracking tools, but it's very attractively presented and not too costly at $1.99.