Both LeafLab's Maple: http://leaflabs.com/devices/maple/
and Diligent's Chipkit Uno32/Max32: http://www.digilentinc.com/Products/Catalog.cfm?NavPath=2,892&Cat=18
Offer Arduino shield compatible boards, with more RAM and much faster processors,
using a very similar IDE and API compatible core libraries.
While the boards are not fully 100% compatible, they can work in many situations.
The maple has its own Arduino compatible IDE.
The maple RET6 board has a 72Mhz 32 bit processor with 512k of flash and 64k of RAM
LeafLab boards also support direct connection to LiPo batteries for easy battery operation.
It sells for a similar price point as the Arduino Mega.
The mpide used for chipkit supports both the AVR Arduino based boards and the chipkit pic32 based boards,
so you can use mpide to build and download sketches for both AVR and chipkit based boards.
This makes it extremely to flip back and forth between different boards using the same sketches.
The chipkit Uno32 sells at a similar price point to the Arduino Uno and the chipkit Max32 sells
at a similar price point to a Arduino Mega.
chipkit uno32 has a 80Mhz 32bit CPU, 128k flash, and 16k RAM.
The chipkit Max32 has a 80Mhz 32 bit CPU with 512k of flash and 128k of RAM.
While both LeafLabs and Chipkit boards have much more CPU power and RAM resources than Arduino Mega
they run at 3.3v and don't have the pin drive capability of the AVR processor.
Both boards are still a bit of "work in progress" as not all the libraries available for AVR based
Arduinos have been ported over to these newer boards yet.
But at least there are some alternatives out there with more horsepower and RAM for a similar price point.