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.
Regarding web.config files...

Web.config is an XML document that contains configuration settings for a web application. It's located in the root directory of a web application, so for SharePoint, it's usually located in the folder C:\Inetpub\wwwroot\wss\VirtualDirectories\SharePoint80, where SharePoint80 is the name of your web application.

Manually making edits to the web.config file is safe as long as you keep the the tags and sections in the right order. Look at [this article](http://msdn.microsoft.com/en-us/library/w7w4sb0w.aspx).

It's also possible to make changes to it programatically, [this post](http://www.crsw.com/mark/Lists/Posts/Post.aspx?ID=32) should show you how.

Hope this helps.