Here is a new one I created. It's not the longest, but I like it because it's iterative/recursive.
http://www.marblerun.at/tracks/615669

I made my counter/sequencer thing using the above tech: http://www.marblerun.at/tracks/615710
Edit: It's not 100% foolproof, but it works 9 times out of ten.

Here's some dank noscopes: http://www.marblerun.at/tracks/615748

Though this in no way affects the gameplay, I find it interesting that the lines on the breakable bricks are randomly generated with each one you place on the board. Plus, they are the only block in the game that you can't click to rotate.

Marble run is really weird, it recorded this one as 111.5 meters.
http://www.marblerun.at/tracks/633694
This one easily reaches over 6,000 meters in a short time.
(I should've put my actual username as the author, but I didn't)


Even more impressive, this one is easily over 20K meters based on my calculations, but it stopped recording at 9,999.
http://www.marblerun.at/tracks/633742
However this employs the use of multiple balls and is not an uncommon idea at all. There's quite a limited range of what you can do in the game, so I think I'm just going to stop here...

http://www.marblerun.at/tracks/805981
somehow i like dis

I made this:/ https://www.marblerun.at/tracks/943646
This https://www.marblerun.at/tracks/943638
https://www.marblerun.at/tracks/943634
https://www.marblerun.at/tracks/943653
https://www.marblerun.at/tracks/942574
https://www.marblerun.at/tracks/942564
https://www.marblerun.at/tracks/942554
https://www.marblerun.at/tracks/942543
https://www.marblerun.at/tracks/942520
https://www.marblerun.at/tracks/942534
https://www.marblerun.at/tracks/941439
https://www.marblerun.at/tracks/940482
https://www.marblerun.at/tracks/940479
and a extra one for the Too ____________ to watch
https://www.marblerun.at/tracks/940455

EDIT:
https://www.marblerun.at/tracks/802708
totally bad

EDIT2:
https://www.marblerun.at/tracks/797531
JUST PUT SPACE IN YOUR NAME BEFORE YOU ASK

I've been trying for the fewest symbols to achieve 999.9m and have so far found a solution with 7 bricks: https://www.marblerun.at/tracks/944808

6 or 5 may be possible, but I haven't found them.

Edit: Found a 6-symbol solution, https://www.marblerun.at/tracks/944946

What a fun toy! Here's my first track: Hornet's Nest

EDIT: 4 bricks, 649.4 meters. Will keep trying.

EDIT×2: Got the 999.9 with 4 bricks!

EDIT×3: Awaiting results for a 3-brick run...

EDIT×4: Here it is.

Typical Weregoose, doing things in unimaginably small sizes.

Clever. Somehow I had been convinced that I'd hit some minimum speed limit and hadn't tried to improve it.

Check out this one I made!
https://www.marblerun.at/tracks/945704
For some reason, it runs different every time. Sometimes the clone block is triggered, sometimes not, but it always gets to the finish.

I totally saw this in this and i totally didn't make it.
https://www.marblerun.at/tracks/994058