Common tasks done with SharePoint Object Model

1. Fetching all the servers in the Farm (We have to add one more namespace for this piece:
Using Microsoft.SharePoint.Administration):
SPSite _site = new SPSite("http://servername:Portnumber/");
foreach (SPServer _server in _site.WebApplication.Farm.Servers)
{
MessageBox.Show(_server.DisplayName.ToString());
}

2.Fetching all the Site Collection in the Web Application.
SPSite _site = new SPSite("http://servername:Portnumber/");
SPWebApplication _webapp = _site.WebApplication;
SPSiteCollection _sitecollection = _webapp.Sites;

foreach (SPSite _IndividualsiteCollection in _sitecollection)
{
MessageBox.Show(_IndividualsiteCollection.Url.ToString());
}

3.Fetching all the sites in a Site Collection.
SPSite _site = new SPSite("http://servername:Portnumber/");
foreach (SPWeb _web in _site.AllWebs)
{
MessageBox.Show(_web.Url.ToString());
}

4.Fetching all the lists in a Site.
SPSite _site = new SPSite("http://servername:Portnumber/");
foreach (SPList _list in _site.OpenWeb().Lists)
{
MessageBox.Show(_list.Title.ToString());
}

5.Fetching all the items in a list.
SPSite _site = new SPSite("http://servername:Portnumber/");
SPListCollection _listCollection = _site.OpenWeb().Lists;
foreach (SPListItem _item in _listCollection[10].Items)
{
MessageBox.Show(_item.Name.ToString());
}

You may also find interesting to read: Important Classes with SharePoint Object Model & Object Model Hierarchy