"Cannot allocate memory" while no process seems to be using up memory

Try this:

1. Go into top.
2. Press F (i.e., Shift+F) to specify the sort field.
3. Type N.  This will turn the lowercase n in the list into an uppercase N.
4. Press Enter.  The list should now be sorted by memory usage.

Press ? or h at the main screen for more options.

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 1010 mysql     20   0  545m  37m 1412 S  0.0  6.3   0:32.97 mysqld
  939 bind      20   0  157m  11m 1896 S  0.0  1.9   0:00.03 named
 3036 ubuntu    20   0 25260 7812 1720 S  0.0  1.3   0:00.42 bash
 1212 root      20   0  192m 7420 2680 S  0.0  1.2   0:03.87 apache2
 1322 www-data  20   0  192m 5880 1088 S  0.0  1.0   0:00.00 apache2
 1323 www-data  20   0  192m 5188  436 S  0.0  0.9   0:00.00 apache2
 1325 www-data  20   0  192m 5188  436 S  0.0  0.9   0:00.00 apache2
 1328 www-data  20   0  192m 5188  436 S  0.0  0.9   0:00.00 apache2
 1329 www-data  20   0  192m 5188  436 S  0.0  0.9   0:00.00 apache2
  968 whoopsie  20   0  197m 4332 2980 S  0.0  0.7   0:00.01 whoopsie
  795 root      20   0  153m 4256 3036 S  0.0  0.7   0:00.06 NetworkManager
 2919 root      20   0 90156 3964 3064 S  0.0  0.7   0:00.02 sshd
  804 root      20   0  101m 3656 2656 S  0.0  0.6   0:00.03 cupsd
 1498 root      20   0  570m 3396 2312 S  0.0  0.6   0:00.03 console-kit-dae
  693 root      20   0 49948 2876 2272 S  0.0  0.5   0:00.01 sshd
  756 root      20   0 79036 2844 2044 S  0.0  0.5   0:00.02 modem-manager
  964 root      20   0  124m 2844 2348 S  0.0  0.5   0:00.02 gdm-binary

ps -e -orss=,args= | sort -b -k1,1n | pr -TW$COLUMNS

will also sort all processes by memory usage descending.