Need a quick fix but only have a web browser? Try FullScreenMario.com. It's Super Mario Bros. cracked open, by creator Josh Goldberg. It's neat seeing this old game in widescreen, but it's even neater to scroll over the level select button and simply click the stage you'd like to warp to. And when you get bored of that, a random button has the game spitting out a fully playable random world instantly!
If you're a creative person, try the map maker. It's super easy to create a world of your own with its drag and drop controls. You can save it as text to pull back up later, too!