I often see offers of free WiFi in cafes, pubs, hotels etc. Are there any security issues I ought to be aware of if I log in and use these?

I could see you watching porn unless you sit in the corner.

Its ok, I'll save the security worries and come and look at your screen.

Dont send anything in the clear. If you are web browsing sites thats OK if you log into a site and it's not using SSL or asks you to accept a certificate then dont, basically dont do anything that involves you typing/sending a username/password.

Aye, wot he said.

Three easy attacks possible:

1) packet snooping: some else on the wifi network listens in on the traffic between you and web servers. This is all plain text unless it is a secure website.

2) man-in-the-middle attack: you end up connected to the web via someone elses PC. When you type in your banks address you just a page on their pc that takes your password.

3) sharing: you leave your file sharing permissions open (e.g. Windows File & Printer Sharing on) and anyone on the network can browse your pc (surprisingly common).

Favourite one in airports is people setting up their PCs to pretend to be wireless access points and then sniffing your email passwords etc.

Don't download anything Mary Whitehouse would have disapproved of
Don't upload ditto
Actually it's a bit broader than than

Don't download anything which any government (however bonkers, eg Britain) might not like

Don't upload anything which any government (however bonkers, eg America) might not like

Confine yourself to messages about how wonderful Michael Gove's free schools initiative is, and you just might avoid Guantanamo Bay

get yourself a VPN. you can get free ones on the interweb.