Also: This.
http://gbatemp.net/threads/which-flash-cart-should-i-get.290097/
Top two recommended are:
Supercard DSTwo (a little over $40, lets you emulate GBA well and SNES with not great, but okay compatibility, best we've got on the DS), has excellent plugin support
R4i Gold 3DS - roughly $20, a little more a little less depending on where you buy, good for DS games and some other emulation
Recommended sites:
zhuzhuchina.com (my favorite)
realhotstuff.hk
nds-card.com (which is experiencing some downtime last time I checked earlier today)
Stay away from Acekard2i as their cart is dead and they are not updating anymore.
Ace3DS has lied to the public saying a v6 update was available, when it is nowhere to be found on the site, and an email a user received from the team confirmed that the fix was not completed. I wouldn't trust them.
The other carts that are not R4i Gold, SCDSTwo, and R4i SDHC are pretty much dead.
