zsIRC has the usual "/cmd" command syntax:
/raw
Sends a raw command to the IRC server.

/join [password]
/j [password]
Joins a channel

/query
Opens a query window to a given user

/nick
Changes your nick

/me
Uh... hard to describe, you should know it already.

/mode [channel] <...>
Same here. The typical /mode command

/part [channel] [message]
Leaves a channel. if you don't specify a channel, it leaves the active one.

/whois
Useless data about a given user

/msg
Sends a simple message

Update Description:
v2008.05.13:
- Removed that annoying beep on Enter for some smartphone devices - thanks to JCO for the (fairly short) testing.

v2008.02.28:
- Fixed: Removed dependency to commdlg.dll, so it runs on more devices.
- Added: Smartphone softkey menu support! It should autodetect the device so you get the best menu you can.
- Updated: Changed update url to use http://zsirc.com.

v2008.02.07:
- FiXato registered zsIRC.com which is currently just a redirect, but I'll make sure it will be pointing here permanently. So use that, it's shorter. Big big thanks for him - it certainly made me realize that people actually care about the client.
- I'm getting sort of bored of the design here, especially because I want to do a proper webforum with logins, etc.
- I seriously need an admin interface for this news. (Yes, I am typing in this as an SQL query as we speak.)
- ...new version of the client would be nice too.

v2007.12.30:
- Another quick bugfix.
- Nicks disappeared sometimes, now it's fixed. (Thanks Robby)

v2007.12.04:
- I've been eyeballing secure connections for a while, but here's a question for someone with a bit more knowledge on the subject. If answered, I might be able to add the feature.

v2007.11.07:
- Quick bugfix.
- Just a quick fix for the semicolon bug in scripting - thanks to AmigaHeret.

v2007.10.21:
- Added automatic triggering. Enjoy.

v2007.05.27:
- Fixed: Timestamp option failed to work in the options dialog.
- Fixed: Stripping of leading ':'.

v2007.04.23:
- Added: .CAB installer.
- Added: Options GUI (!!!).
- Fixed: Some servers/proxies return only \n instead of \r\n.

v2007.03.03:
- Fixed: Added option for offscreen buffering - read .ini for details!!
- Fixed: ACTION in query windows.

v2007.02.27:
- Fixed: Flickering.
- Fixed: ForegroundColor didn't work.
- Added: Codepage parameter in .ini.

v2007.02.11:
- Fixed: Removed silly deprecated VERSION reply.
- Fixed: Resolving code was broke with IP-s.
- Added: ForegroundColor parameter in .ini (DOH!).
- Added: StripColorCodes parameter in .ini.

v2006.10.28:
- Fixed: Bad logging format.
- Fixed: Space delimiting.
- Added: PassCommandsRaw parameter in .ini.
- Added: Ctrl-enter disregards leading slash.
- Added: WHOWAS reply.

v2006.09.09:
- Fixed: Forced disconnect broke reconnect.
- Fixed: Cut/copy to clipboard (sheesh).
- Added: Temporary copy solution - click on a line to copy the entire line to clipboard (I know, not perfect, but this is the most I can provide right now).

v2006.07.23:
- Fixed: Multi-line /names.
- Fixed: Weird crashbug on disconnect.
- Fixed: Empty notices (--).
- Fixed: Moving down in the buffer preserves previous line.
- Fixed: Linebreak after session closed in logs.
- Fixed: double popup of context-menus.
- Fixed: Editbox didnt adapt to rotation.
- Added: /invite.
- Added: Nick-complete on TAB.
- Added: stuff.

v2006.07.20:
- Fixed: Word-wrap bug that caused zsIRC to hang up completely.
- Fixed: Bug in settings loader caused server name to reset.
- Fixed: Autoconnect didn't work (jeez).
- Fixed: Empty notify list didn't compute (JEEZ).
- Added: Support for BACK button on Smartphones (Please test/try this and report to the forum, I only have one device).

v2006.07.12:
- Fixed: Weird JOIN's without delimiting colon.
- Fixed: Much better wordwrap.
- Added: Multi-line paste.
- Added: Notify list.

v2006.07.07:
- Added: Rewrote setting system, added /set.
- Fixed: Fixed the networking.
- Fixed: Timestamp parsing.
- Fixed: Nickchange made user lose op/voice.
- Added: Version info to about + introductory text.
- Added: Update check + homepage link.
- Added: Autoconnect.
- Added: Server password.
- Added: Option to close windows or rejoin.

v2006.06.13:
- First public beta released!
- Added: Various server message handlings.
- Added: /mode, /whois, /msg, /part.

v2006.06.11:
- Added: /me, /nick, ...
- Added: Topic changes.
- Added: Context-menus.
- Fixed: Network handling.

v2006.06.04:
- Added: 10 lines of buffering.
- Fixed: Ugly bug with packet handling.
- Fixed: Logging into invalid filenames.

v2006.05.29:
- Added: Loading settings (.ini files, edit with Pocket Word or something).
- Added: MODE-handling.

**** [Jul 19, 2006] by Benway using MDA Pro
Okay, the software's been updated, and now it works correctly on the MDA Pro. Though it has fewer features than the commercial wmirc irc software, the layout is more like mirc, with a chatters list on the right. wmirc is probably more useful if you're opping in a busy channel, coz it allows predefined commands, but for basic chatting, this is nicer.

** [Jul 3, 2006] by Benway using MDA Pro
Sadly, this doesn't work properly on the MDA Pro. After a little tinkering, I was able to join my favourite channel, but everyone's text appeared in individual tabbed windows (as if they were senfding me pvt msg's), even though they were actually speaking in the open channel.