Answer: Here is a sample use of a helper function (VB.Net)

<asp:DataGrid id="YourID" runat="server" AutoGenerateColumns="False">
<Columns>
<asp:TemplateColumn HeaderText="Status">
  <ItemTemplate>
    <%#ShowStatus(Container.DataItem("online"))%>
  </ItemTemplate>
</asp:TemplateColumn>

Function ShowStatus(val)
  If val = 0 Then
    ShowStatus = "Offline"
  Else
    ShowStatus = "Online"
  End If
End Function

Updated! Answer: Here's an example of how to set the width of an edit Textbox in the EditItemTemplate of your TemplateColumn:

<asp:DataGrid id="YourID" runat="server" AutoGenerateColumns="False">
<Columns>
<asp:TemplateColumn HeaderText="Sample Column">
  <ItemTemplate>
    <%#Container.DataItem("AColumnName")%>
  </ItemTemplate>
  <EditItemTemplate>
    <asp:Textbox runat="server" width="600" maxlength="600"/>
  </EditItemTemplate>
</asp:TemplateColumn>

And for a BoundColumn, you can modify the width just before the Datagrid is rendered:

Private Sub DataGrid1_PreRender(s As Object, e As EventArgs)
If DataGrid1.EditItemIndex > -1 Then
  Dim tbx As TextBox
  tbx = CType(DataGrid1.Items(DataGrid1.EditItemIndex).Cells(0).Controls(0), TextBox)
  tbx.Width = Unit.Parse("1cm")
End If
End Sub

Answer: AutoGenerated columns do not appear in the Datagrid's Columns() collection, and so the usual method of hiding a Datagrid column will fail:

'Will NOT work for AutoGenerated columns:
Datagrid1.Columns(1).Visible = False

So the place to handle this is in the ItemDataBound event of the Datagrid:

<asp:DataGrid id="Datagrid1" runat="server" AutoGenerateColumns="True" OnItemDataBound="Datagrid1_OnItemDataBound"/>

Private Sub DataGrid1_ItemDataBound(s As Object, e As DatagridItemEventArgs)
  e.Item.Cells(1).Visible = False
End Sub

Answer: This is caused by calling DataBind() on the grid before retrieving the updated values. The DataBind() overwrites the updates with the original values. Usually caused by having the initial DataBind() of the grid called on every Page_Load. Solution: call DataBind() only the first time the page is loaded:

Sub Page_Load(s As Object, e As EventArgs)
  If Not IsPostBack Then BindGrid()
End Sub

Answer: Handle this in the ItemDataBound event, where you can access the DataItem as it is bound to the Datagrid, and the Cells() collection of the current Datagrid item.

Sub Datagrid1_ItemDataBound(source As Object, e As DataGridItemEventArgs)
  If (e.Item.ItemType = ListItemType.Item Or _
    e.Item.ItemType = ListItemType.AlternatingItem) Then
    If Convert.ToDateTime(e.Item.Cells(1).Text) < DateTime.Today Then _
    e.Item.Cells(1).BackColor = System.Drawing.Color.FromName("#ffccff")
    If e.Item.DataItem("UserID") = 590 Then _
    e.Item.Cells(2).BackColor = System.Drawing.Color.DeepPink
  End If
End Sub

Answer: The HyperlinkColumn's DataNavigateUrlFormatString only supports one parameter. If you need more than one, switch to a TemplateColumn with an <asp:Hyperlink>.
Note: You should always encode values passed into querystring to protect against spaces and special characters.

<asp:DataGrid id="YourID" runat="server" AutoGenerateColumns="False">
<Columns>
<asp:TemplateColumn HeaderText="Sample Column">
  <ItemTemplate>
    <asp:Hyperlink runat="server" Text='<%#Container.DataItem("TextVal")%>' NavigateUrl='<%# "page.aspx?Param1=" & Server.UrlEncode(Container.DataItem("Val1")) & "&Param2=" & Server.UrlEncode(Container.DataItem("Val2"))%>'/>
  </ItemTemplate>
</asp:TemplateColumn>
</Columns>

Answer: Use a TemplateColumn, and inside the ItemTemplate (or EditItemTemplate), you can use <br> tags or any other formatting you might want.

<asp:DataGrid id="YourID" runat="server" AutoGenerateColumns="False">
<Columns>
<asp:TemplateColumn HeaderText="Sample Column">
  <ItemTemplate>
<%#Container.DataItem("Field1")%>
<br>
<%#Container.DataItem("Field2")%>
  </ItemTemplate>
</asp:TemplateColumn>
</Columns>