Here are some configurations for various providers with Asterisk. They worked for me, they might not work for you. Also, they may not be the best way to configure Asterisk, or the most secure way, or the most efficient way, so use these configuration notes at your own risk. They're also mostly geared towards basic connectivity (as opposed to being part of some grand dialplan) - hopefully it'll be enough information to get you started. A lot of these are based upon (or inspired by) the configurations found at voip-info.org (which is the best resource on VOIP I've found).
One other thing - check your provider's terms and conditions. It may be against your providers terms and conditions to use any non-authorised software - I will not be responsible for what happens to you if you break any terms and conditions. While it's extremely silly that a provider would choose to use an open standard and then require their users to only use approved software (it makes about as much sense as having a website and only allowing people to use the providers own web browser), we have to remember that we live in a world where corporations sometimes display odd behaviour.