If you have a GameBoy Advance, they've released SMB3. If you don’t have a GBA, find a GBA emulator on the net (I use VisualBoy Advance). Then it’s just a matter of finding a SMB3 ROM for your emulator (your best luck is probably with mIRC).
Personally, I loved SMB2 best of all.
For all of you Mario fans, I would also suggest the recently released Mario vs. Donkey Kong. Nifty little puzzle game which I spent most of yesterday playing. And to show how much SMB2 has become part of the franchise, the game includes many elements of SMB2.
To get the most out of SMB3 for GBA, don't you have to use that damn E-Reader?
I'd stick with my Super Mario World + All Stars cart for now.
I liked SMB2 as a fun little diversion, but I don't think it comes anywhere near most of the other entries in the series.
Mario vs. DK is a semi-sequel to the 1994 Donkey Kong on GBA, which, yeah, borrowed elements from SMB2.