Memory in ps
USER PID %CPU %MEM VSZ RSS STAT TIME COMMAND
akkana 4074 0.4 2.2 187628 68920 Sl 00:00:10 firefox-bin
root 3966 0.4 0.7 30324 21988 Ss+ 00:00:08 Xorg
akkana 4053 0.0 0.6 33104 18624 S 00:00:01 xchat
akkana 5221 0.0 0.5 25568 16344 S 00:00:00 emacs
akkana 4063 0.0 0.4 27876 13060 Sl 00:00:00 python
akkana 4038 0.0 0.2 12360 7372 S 00:00:00 openbox
root 3720 0.0 0.2 21376 6584 Ss 00:00:00 apache2
akkana 4598 0.0 0.1 8400 4628 S+ 00:00:00 mutt
105 3410 0.0 0.1 6660 4468 Ss 00:00:00 hald
akkana 4032 0.0 0.1 9232 3652 S 00:00:00 xterm
www-data 5935 0.0 0.1 21376 3444 S 00:00:00 apache2
www-data 5936 0.0 0.1 21376 3444 S 00:00:00 apache2
www-data 5937 0.0 0.1 21376 3444 S 00:00:00 apache2
www-data 5938 0.0 0.1 21376 3444 S 00:00:00 apache2
www-data 5939 0.0 0.1 21376 3444 S 00:00:00 apache2
akkana 4132 0.0 0.1 8796 3356 S 00:00:00 xterm
akkana 4077 0.0 0.1 5292 3104 S 00:00:00 gconfd-2
RSS: amount of memory resident in memory
(not swapped to disk).
It's not a perfect measure, but it's a start.
Units are kilobytes.