The #nslu2-general IRC channel on irc.freenode.net has been established for end-user questions. You can view the Live IRC Logs for that channel.
Most of the core development discussion happens in the #nslu2-linux IRC channel on irc.freenode.net - note that end-user questions are not welcome on this channel. You can view the IRC Logs for that channel (they are delayed by 24 hours). We also have Live IRC Logs online which cover the last 24 to 48 hours.
SlugOS (formerly known as OpenSlug)
The OpenSlug firmware has its own #openslug IRC channel on irc.freenode.net. You can view the Live IRC Logs for that channel. If you are using the DebianSlug firmware without using the Debian packages, then you should also use the #openslug channel (it really should be renamed to #slugos now).
If you are using the Debian packages (either little-endian or big-endian), then you should use the #debian-arm channel on irc.oftc.net instead.
The firmware for the Synology DS-101(g+) has its own channel (#ds101-linux) on irc.freenode.net. You can view the Live IRC Logs for that channel.