*List of useful diagnostic tools*

Generic tools available on a linux system
Process/memory/load
ps
top, atop, htop
uptime: for load and uptime
free or cat /proc/meminfo
dstat
pmap
pgrep
sar


IO/interrupts
vmstat
iostat
iotop
sar

System call/file descriptors
strace
lsof
ltrace
fuser
oprofile
systemtap
ipcs


Logs
grep
tail
multitail: follow multiple files
ack: package name is ack-grep in debian



Network
iftop
tcpdump
wireshark
ethtool
ngrep
netcat
netstat
vnstat
iptraf
ifstat
nmap
mtr
dnstracer
sar

Filesystem
ls
find
du
df
file


Binaries
gdb
elfutils
frysk
ldd
strings


Language specific tools

Java
jstat
jmap
jps
jconsole/jvisualvm
memory analyzer: http://www.eclipse.org/mat/

Python
pdb

Ruby
ruby-debug
xray (allows you to send kill -3 to your ruby process)

PHP (debugging and profiling)
xhtprof
xdebug


Software specific

MySQL
mytop
maatkit
innotop
mysqltuner


Virtualization (libvirt)
virt-top