It would be awesome if Sparkfun could produce a couple more shields for the Arduino.
Relay Sheild - have something like 4 little relays connected to 4 of the Digital’s, to switch larger items such as fans, lights, or maybe even AC (with care)…
Opto Sheild - have an opto isolated chip with say 4 channels going to 4 of the digitals, so noisey or potentially spikey inputs can trigger the Arduino inputs.
Mega Sheilds - Not only are Mega’s expensive, but they lack sheilds in a big way. I know personally this is what is stopping me buying my first Mega. If Sparkfun could start the ball rolling, and potentally make a mega sheild or two, that would be awesome. I know some sheilds do work on the mega, but a number dont. And the ones that do, some need pins bent out in order to get them to work. A proper mega shield would be a winner I think.
Development Boards - I know there are a few Olimex boards for AVR’s, however they are a little dated in my opinion. I really like the Mega16 board with the relays and opto isolated inputs - however what puts me off is that its a Mega16 and not something else. If a development board could be made with say the AtMega644P, that would be totally awesome. It could then have a serial port, and a FDTI port, so effectively it could be uploaded with a Sanguino bootloader or another variation of the bootloader and used with Arduino IDE. Also feature the 10 pin standard AVR programming header. I think these would sell really well. The ATMega644P is a great chip, and now that its been ported to Arduino its even more appealing.
Just my 2c 
J