A new feature was just added that allows you to create custom modes! I've been enjoying experimenting with play modes and figured you would too. All you need to do is edit the song tag to include the special keyword(s) you want. Here's some examples of what you can do:
EXAMPLE 1: This one simply requires Mono characters. By including this keyword you'll be able to compete against Mono players only. Edit the song's tag so that instead of just being "Songtitle" it is "Songtitle [as-monoonly]" (without the quotes).
EXAMPLE 2: This one greatly buffs up scoring potential for Mono characters and also removes the stealth and cumulative hit bonuses. Elite and Pro Monos will have as many color blocks as Casual Mono usually has and each block will have a base score of 4 instead of the usual 3.
"Songtitle [as-lessgrey] [as-monopt4] [as-nostlth]"
EXAMPLE 3: You've composed a great song for Audiosurf and want to send it to a friend, but you want them to have an extra wild ride. This turns the camera sideways and uses many additional strings of blocks.
"Songtitle [as-swind] [as-caterp]"
EXAMPLE 4: This is a tricky mode I enjoyed experimenting with. The puzzle grid is only 5 rows tall and matches don't collect until there's 6 of a color touching. Matches take 6 seconds to collect. Also, traffic is flooded with black/white blocks.
"Songtitle [as-msz6] [as-wb25] [as-mt150] [as-prows5]"
EXAMPLE 5: Mono (even Double Vision Mono) with no greys. Try and hit every block.
"Songtitle [as-everybodymono] [as-nogrey]"
EXAMPLE 6: Hidden puzzle mode with 4 lanes. The ultimate memory challenge.
"Songtitle [as-4lane] [as-hidepuz]"
Here's a list of all the available keywords for you to experiment with however you want. For the ones with an X, put a number there (like 3 or 127). You can uses many at a time, but only one from each group can be used. Note that these keywords are removed before Audiosurf sends the song's title to last.fm.
[as-4lane] Play with 4 lanes
[as-monoonly] Mono characters required
[as-everybodymono] All characters have only 2 block colors
[as-lessgrey] Mono characters have less grey blocks
[as-allgrey] Mono all greys. Scores start at 1000 and go down with each hit.
[as-nogrey] Mono characters have no grey blocks
[as-monoptX] Base points for Mono characters (default 3)
[as-nostlth] Remove Stealth and cumulative hit bonuses
[as-swind] Sidewinder camera mode
[as-bankcam] Banking camera mode
[as-first] First-person camera mode
[as-caterp] Big strings of blocks instead of only solo blocks
[as-mszX] Minimimum match size (number for X)
[as-wbX] Percentage of white/black blocks
[as-mtX] Match collection ticks (default 10-25)
[as-prowsX] Puzzle row count (0-8)
[as-hidepuz] Play with an invisible puzzle grid
[as-steep] Tries to make more exciting rides for uphill songs
It's not required, but to have a better chance of finding populated scoreboards for your custom modes follow the conventions set in the examples:
-Put spaces between the brackets.
-Keep your keywords in the order listed above. If you want to use both "[as-everybodymono]" and "[as-caterp]", make "[as-everybodymono]" the first one since it is higher up on the list.
Please share the custom modes you like! You'll probably want to try and stir up some competition on those scoreboards too.
EDIT: As noted in the comments below there's a really quick way to change an MP3's song tag. Just right click the file and choose "Properties". Then select the "Summary" tab and edit the song's title there.
EDIT 3.5.09: Added the "[as-steep]" tag