To capture traffic on a Cisco ASA or PIX firewall the capture command can be used.

Example: Capturing traffic on ASA/PIX

You want to capture traffic from/to host 10.100.100.1 located behind the dmz interface.

The access-list is optional and is used to filter to interesting traffic
pix(config)# access-list interesting_traffic permit ip host 10.100.100.1 any
pix(config)# access-list interesting_traffic permit ip any host 10.100.100.1
pix(config)# capture cap1 access- interesting_traffic interface dmz

pix1(config)# show capture
capture cap1 access-list access-interesting interface dmz

Commands to show capturing results:
show capture cap1
show capture cap1 detail
show capture cap1 dump

Command to clear captured traffic:
clear capture cap1

Command to save results to tftp server:

copy capture:cap1 tftp://10.1.1.1/dmzhost.txt

To save results in pcap format:
copy capture:cap1 tftp://10.1.1.1/dmzhost.txt pcap

Command to disable capturing:

pix(config)# no capture cap1

This can be very helpful in troubleshooting connectivity issues.  I most recently used this to troubleshoot VoIP issues for a customer.