After you new a tomcat server in eclipse and use worspace metadata(does not modify Tomcat installation), you may see 404 error when accessing localhost:8080/, you need to do the following thing:
Copy the ROOT (Default) Web App into Eclipse.
Eclipse forgets to copy the default apps (ROOT, examples, etc.) when it creates a Tomcat folder inside the Eclipse workspace. Go to C:\apache-tomcat-7.0.34\webapps, R-click on the ROOT folder and copy it. Then go to your Eclipse workspace, go to the .metadata folder, and search for "wtpwebapps". You should find something like your-eclipse-workspace\.metadata\.plugins\org.eclipse.wst.server.core\tmp0\wtpwebapps (or .../tmp1/wtpwebapps if you already had another server registered in Eclipse). Go to the wtpwebapps folder, R-click, and paste ROOT (say "yes" if asked if you want to merge/replace folders/files). Then reload http://localhost/ to see the Tomcat welcome page.
Installing Tomcat 7 and Using it with Eclipse