another day, another piece of news about the leaking of email account passwords. this time it's a yahoo server that's been hacked, but the leak includes addresses from gmail and hotmail among other domains.
the security company involved have created a tool where you can check to see if your address was involved in the leak - http://labs.sucuri.net/?yahooleak
for a more general check to see if your password is known to have been compromised, go to https://shouldichangemypassword.com/. some would argue that you should change your password every so often anyway, and this is certainly the requirement in many workplaces. however others argue that such policies either offer no benefits, or actually reduces security - see for example this discussion on an IT security professionals website.
at the very least, use a strong password, and consider using a password manager to help you manage all the various strong passwords you have around the place. i use UPM - universal password manager, and have found it suitable for my needs.