Note: args in <triangles> are required; args in [brackets] are optional.
List of commands:

music
Gives descriptions of music commands, sometimes linking to a help thread on SMW Central.
!ar music [term]

sound
Shows the song/sound effect at the given address.
!ar sound <address>

findsound
Search feature for the song/sound list.
!ar findsound <terms>

13bf
Switches a level ID between LM and 13BF format.
!ar 13bf <level>

xram
Like ram, but for a few obscure addresses that aren't in ram for reasons omnomnom'd.
!ar xram <address>

addr
Switches between SNES and PC addresses. Can be used as an alternative to Lunar Address.
!ar addr <address>

oam
Takes an oam value from ram 0200 or 0420 and returns the opposite value. Really only useful if you're directly poking those addresses.
!ar oam <value>

sprite
Returns the name (and extra values, when applicable) of the sprite in question.
!ar sprite <id>

findsprite
Searching tool for sprite.
!ar findsprite <terms>

gfx
Shows the GFX list.
!ar gfx

findgfx
Searching tool for gfx
!ar findgfx <terms>

as
Assembles the given opcodes, returning raw hex. It's rather low-end and errors for some oddball opcodes and/or fancy stuff.
!ar as <opcodes>

cycle
Tells how long a given piece of code takes to execute. Same assembler as !ar as, but can also take raw hex. Doesn't follow branches, it just counts each opcode once.
!ar cycle <opcodes>

dis
Disassembles raw hex into its opcodes.
!ar dis <hex>

eval
Executes a piece of 65816 ASM or machine code.
!ar eval <opcodes>

db
Builds an ASM table define. dw, dl and dd are also available.
!ar db <address>

th
Takes a text string and converts it into hex.
!ar th <string>

unhex
Converts a hex string into text.
!ar unhex <hex>

th16
Takes a text string and converts it into 2-byte Unicode.
unhex16 converts it back into text.
!ar th16 <string>

unhex16
Converts a 2-byte unicode hex string back into text.
!ar unhex16 <hex>

alllog, allog
Links to a all.log, a SMW disassembly. Contains both the original version and the smwdisc version.
!ar alllog

ascii
Links to text files that show hex values used to show letters in SMW.
!ar ascii

bgrip
Links to a page that allows you to rip a BG from given file(s).
!ar bgrip

cgi
Links to an online version of AlcaRobot. Not all commands are supported in the online version.
!ar cgi

tile
Links to list of Map16 tile IDs and their descriptions.
!ar tile

pose
Links to a list of Mario's poses in SMW and their hex values. Used with RAM address $13E0.
!ar pose

rats
Posts a text file that illustrates RATS tags.
!ar rats

reg, regs
Links to a text file containing a list of SNES registers. A second argument may be given to describe the given register.
!ar reg [address]

findreg, findregs
Works the opposite way of reg; this command searches the descriptions of SNES registers and shows (or links if too many) the desired address(es).
!ar findreg <search terms>

jslrts
Shows how to JSL to a routine ending in RTS.
!ar jslrts [address]

col
Switches between PC (rrrrrrrrggggggggbbbbbbbb) and SNES (0bbbbbgggggrrrrr) colors.
!ar col <color>

render
Renders one or more SNES colors.
!ar render 0000 7fdd 0000 32b7 67fb 0200 0320 03e0

strlen
Shows the length of the given text string.
!ar strlen <string>

math
Computes the given mathematical statement. 
!ar math <equation>

wmath
Sends the input to Wolfram|Alpha, which will read and compute it. W|A can do many things including math, unit conversions and making funky graphs.
!ar wmath <input>

trans
Translates a string of text using Google Translate.
!ar trans [language of origin]|[target language] <string>
Both languages are optional. If the first one is omitted, Google is told to guess the language. If the second one is omitted, English is assumed.
- and , and ,, are also valid separators. If using , you must specify target language, or the bot assumes the comma is a word (examples: "So, you said you're twelve?", "En, två, tre").

kat/hir
Converts roomaji to katakana/hiragana. 
!ar hir <roomaji>

dehir
Converts katakana/hiragana to roomaji. (katakana become ALLCAPS).
!ar dehir <katakana|hiragana>

yxppccct
Shows flags set in the given hex value or shows the hex value of the given yxppccct (caps on a letter meaning that bit is set)
!ar yxppccct [hex]

help
Shows this file. Given that you probably used it to get here, you really shouldn't expect anything else.
!ar help