The NDZeit TubeBaby is a pretty basic emulation but with several different amp types including American, British and Custom.

“TubeBaby is a digital simulation of a tube guitar amplifier. As such it features the standard controls found on a real amplifier, namely gain, bass, mid, treble, presence, master and a switch for activating the high gain mode of the preamp stage. The latest version also features a menu that lets you choose between three amplifier types, five cabinet emulations and the amount of high frequency passing through the power amp section. The menu can be accessed by clicking on the TUBEBABY logo in the upper left corner of the plugin.
TubeBaby is available as a freeware vst plugin for windows and can be used in every host application that supports vst plugins”

Download: http://www.ndzeit.org/guitar/tubebaby.html