Memory

Testing RAM for errors
Memtest86 is thorough, stand alone memory test for x86 architecture computers. It is offered in a ISO image, so that you can put the image on bootable CD.

download memtest

Debugging memory hogs in BSD
To discover which process is using the most memory percentage wise:

USER   PID %CPU %MEM   VSZ  RSS  TT  STAT STARTED      TIME COMMAND frankk 64873 0.0  7.9 71352 61644  ?? S   11:17PM  20:07.78 /usr/X11R6/lib/firefox/firefox-bin frankk  607  0.0  7.7 74080 59832  ?? S    4:25PM   6:18.70 /usr/X11R6/lib/thunderbird/thunderbird-bin frankk  583  0.0  7.0 64728 54568  ?? I    4:25PM  17:46.62 /usr/X11R6/lib/firefox/firefox-bin root   561  0.8  3.0 152640 23116  v0  R     4:25PM  10:37.38 X :0 -nolisten tcp (Xorg) frankk  869  0.0  3.7 34332 29036  ?? S    6:10PM   6:13.38 gaim frankk  611  0.0  1.6 16476 12560  ?? S    4:28PM   1:39.09 xchat root   678  0.0  0.5  5604 4000  ?? S    4:54PM   0:05.35 xterm (xterm-static) root   644  0.0  0.5  5512 3916  ?? S    4:35PM   0:14.46 xterm (xterm-static) root   392  0.0  0.3  3460 2332  ?? Ss   4:24PM   0:01.97 sendmail: accepting connections (sendmail)
 * 1) ps aumx | head

Two other columns worth noting:

vsz  =     virtual size in Kbytes (alias vsize)

rss  =    resident set size

Lookup "KEYWORDS" in the ps Manual for a list of keys you can display.

swap is also an important part of memory that resides on the disk.