Question: I have a website in Sharepoint 2007. We use a wsp to deploy our projects. We can deploy dlls, usercontrols, features, but i don't know how to include new lines in the web.config.

Which would be a possible way to make changes in web.config?

Another thing, how can I include resources (resx) in the wsp?

Answer: 
Manually editing the web.config in a multi-server farm is a Bad Idea. Ensuring the web.config files stay in sync on each WFE will become a nightmare quickly.

Using the SPWebConfigModification class to manage web.config changes across the farm is a Good Idea, as changes will be persisted in the SharePoint configuration database, and automatically pushed to every server in the farm.

Having said that, coding against the class can be a pain. Luckily there's a [pretty good front-end](http://spwebconfigmodif.codeplex.com/) that's freely available, giving you a configuration screen in Central Administration to add/remove web.config changes.
I think direct web.config manipulation can get tricky when you have multiple servers. If your entire installation is only on a few servers, it just might be easier doing that than trying to make simple web.config changes via code.

Plus, manipulating the web.config directly ensures you have a complete web.config in your source control system. If you use the API to make changes, all you'll have in source control is a bunch of C# code that you hope are bug free!