Some handy one-liner shell scripts.
Note: if you're on Windows, all of these tools are available 
as part of the free Cygwin tookkit (www.cygwin.com).
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# Delete all jobs matching the pattern "job=DELETEME*".
p4 jobs -e "job=DELETEME*" | cut -d " " -f 1 | xargs -n 1 p4 job -d

# List all changelists that contributed via integration to target change CHANGE.
p4 files @CHANGE,@CHANGE | sed s/#.*/@CHANGE/ | p4 -x - filelog -m1 | grep "^\.\.\. \.\.\." | grep -v "into" | grep -v "ignored by" | sed "s/.*\/\//\/\//" | p4 -x - changes

# List all files opened on any client owned by USER, without using p4 opened -a.
p4 -Ztag clients | grep -B3 "... Owner USER" | grep "... client" | cut -d ' ' -f 3 | xargs -n1 p4 opened -a -C

# Delete all empty submitted changelists.
p4 changes -s submitted | cut -d " " -f 2 | xargs -n1 p4 change -d -f

# Not quite a one-liner, but almost.  Prevents new user creation by putting
# all current users into a group and then changing "user *" to "group *".
p4 users | cut -d" " -f1 | sed -e "s/^/\t/" > users.txt
p4 group -o everyone | cat - users.txt | p4 group -i
p4 protect -o | sed -e "s/user \*/group \*/" | p4 protect -i

# Returns output if and only if USER has access to FILE.  (Doesn't necessarily work with wildcards.)
p4 protects -u USER FILE | tail -n1 | grep -e " //"