Paul Schaeflein did a post about an example how to programmatically upload a file to a document library. The example is in VB.NET and could be very handy to use in custom made WebParts. Thanks for that! :)

Sub ProcessPostedFile(ByVal fileUpload As HtmlControls.HtmlInputFile)
    Try
    ' get the filename and stream
    Dim fn As String = System.IO.Path.GetFileName(fileUpload.PostedFile.FileName)
    Dim stm As System.IO.Stream = fileUpload.PostedFile.InputStream
    Dim contents(CInt(stm.Length)) As Byte

    stm.Read(contents, 0, CInt(stm.Length))
    stm.Close()

    ' get the library path
    Dim docLibPath As String = Configuration.ConfigurationSettings.AppSettings("DocUploadPath")
    ' first, get the site containing the library
    Dim site As SPSite = New SPSite(docLibPath)
    Dim web As SPWeb = site.OpenWeb
    ' then get the folder
    Dim docFolder As SPFolder = web.GetFolder(docLibPath)

    ' delete the file if it exists
    Dim docFile As SPFile
    Try
        docFile = docFolder.Files(fn)
    Catch ex As Exception
    End Try

    If Not docFile Is Nothing Then
        docFolder.Files.Delete(fn)
    End If

     ' save the file
    docFile = docFolder.Files.Add(fn, contents)

    Catch ex As Exception
        Context.Trace.Warn(ex.ToString)
    End Try
End Sub

http://sharepointblogs.com/SPDiary/archive/2005/07/12/2632.aspx

Article written by Bil Simser doing the same upload using a HTTP PUT. Meaning that you could use it from any place and not only on the SharePoint server.

http://weblogs.asp.net/bsimser/archive/2004/06/06/149673.aspx