The copy command can be very useful for quick operations. However, there are a number of tricks that are not widely known.

1) Copying the contents of a source directory into the current directory: just specify the source directory!

y:\privates>copy C:\WINDOWS\system32\clients\twclient\x86
C:\WINDOWS\system32\clients\twclient\x86\twcli32.msi
        1 file(s) copied.

2) Creating a text file without an editor:

y:\privates>copy con sample.txt
This is a text file.
Another line...
^Z
        1 file(s) copied.

y:\privates>type sample.txt
This is a text file.
Another line...

3) Creating a zero-size file

C:\>copy nul empty.txt
        1 file(s) copied.

C:\>dir empty.txt
 Volume in drive C has no label.
 Volume Serial Number is FCCD-E1D0

 Directory of C:\

02/26/2005  10:38 PM                 0 empty.txt
               1 File(s)              0 bytes
               0 Dir(s)  24,429,268,992 bytes free

4) Appending to a text file. Note that /A flag

y:\privates>copy /A abc.txt + con
abc.txt
con
BBB
^Z
        1 file(s) copied.

y:\privates>copy /A abc.txt + con
abc.txt
con
CCC
^Z
        1 file(s) copied.

y:\privates>type abc.txt
AAA
BBB
CCC

[update: item (3) above is fixed now.]