mIRC: /server -n opens a new server window, /server -m ntfs.org opens a new server window and connects, in this case, to ntfs.org.
X-Chat: /newserver ntfs.org opens a new server window to ntfs.org.
BîtchX (And possibly epic?): /window new hide
/window next
/server +ntfs.org will open a new hidden window, switch to it, then add ntfs.org to your open servers
With most clients, there will be "new window" or "new server" under the file menu or in the options somewhere.