I've collected a few short (usually one stage) hacks that I believe are notable for one reason or another. The zip archive includes the following:
Bokoboko 1
Bokoboko 2
These are pretty easy hacks that give you a good feel for SMB glitch hacks in general.
Dondokodon Mario
You can't run in this hack, and it makes extensive use of one particular glitch. See if you can figure it out.
Narrow Way, by Snowarrow
This is like Bokoboko, but taken to a ridiculous extreme. It's very hard, and takes a lot of walljumping to complete.
Introductory Mario 1, by U1
Introductory Mario 2, by U1
These are wonderful introductions to the many, many glitches of Super Mario Bros., and is very helpful for beginning SMB players and hackers.
Force, by Rei
A pretty ridiculous hack that requires lots of straight-up wall entering. Very difficult; not for the faint of heart.
Mario is Erotic, by Snowarrow
The title is completely nonsensical, but it really tests your ability in performing the most difficult SMB glitch.
Download(Patch to "Super Mario Bros. (JU) [!].nes")