proc tk_getString {w var title text} {
    variable ::tk::Priv
    upvar $var result
    catch {destroy $w}
    set focus [focus]

    toplevel $w -bd 1 -relief raised -class TkSDialog
    wm title $w $title
    wm iconname $w $title
    wm protocol $w WM_DELETE_WINDOW {set ::tk::Priv(button) 0}

    entry $w.entry -width 20
    button $w.ok -bd 1 -text Ok -default active -command {set ::tk::Priv(button) 1}
    button $w.cancel -bd 1 -text Cancel -command {set ::tk::Priv(button) 0}
    label $w.label -text $text

    grid $w.label -columnspan 2 -sticky ew -padx 3 -pady 3
    grid $w.entry -columnspan 2 -sticky ew -padx 3 -pady 3
    grid $w.ok $w.cancel -padx 3 -pady 3
    grid rowconfigure $w 2 -weight 1
    grid columnconfigure $w {0 1} -uniform 1 -weight 1

    bind $w <Return> {set ::tk::Priv(button) 1}
    bind $w <Destroy> {set ::tk::Priv(button) 0}
    bind $w <Escape> {set ::tk::Priv(button) 0}

    wm withdraw $w
    update idletasks
    focus $w.entry
    set x [expr {[winfo screenwidth $w]/2 - [winfo reqwidth $w]/2 - [winfo vrootx $w]}]
    set y [expr {[winfo screenheight $w]/2 - [winfo reqheight $w]/2 - [winfo vrooty $w]}]
    wm geom $w +$x+$y
    wm deiconify $w
    grab $w

    tkwait variable ::tk::Priv(button)
    set result [$w.entry.entry get]
    bind $w <Destroy> {}
    grab release $w
    destroy $w
    focus -force $focus
    update idletasks
    return $::tk::Priv(button)
 }

usage:

 if {[tk_getString value "Dialog" "Please enter some text"]} {
    puts "user entered: $value"
 }