Here are some useful commands if you're toying around with Varnish and want to get some more insight into what's happening. Varnishtop will show you the hits you're getting in a "top"-like style, so the most active hits are shown on top. Varnishlog simply iterates over all logs and displays them as you would tail a normal log-file.
Top requests to your backend
This shows only the "TxURL", the URL being retrieved from your backend.
# varnishtop -i TxURL # varnishlog -b | grep 'TxURL'
Requests made by a client
This shows only the "RxURL", the URL being requested.
# varnishtop -i RxURL # varnishlog -c | grep 'RxURL'
Show cookies being sent by clients
This will list all HTTP headers with a "Cookie" keyword in the request that are being sent to Varnish.
# varnishtop -i RxHeader -I Cookie # varnishlog -c | grep 'Cookie: '
Which host is being requested the most
Only really useful when you're serving multiple hosts in Varnish.
varnishtop -i RxHeader -I '^Host:' varnishlog -i RxHeader | grep 'Host: '
There's plenty more useful & small Varnish commands, if you have some more feel free to add them to the comments!