Polarity's new Tubester 1
is a freeware VST plugin combining preamp, EQ, and guitar speaker
simulator. AmpliTube it ain't, but if you need a quick-and-dirty tube
amp plug with low CPU overhead, this could be the trick. There's no
custom UI; it uses whatever your host supplies. (via Harmony Central)

While you're there, check out Riffster Lite,
a free app for transcribing phrases from music, with variable tempos
and phrase selection. The pro version is US$24.99 and adds more tempo
options, EQ, instrument cancelling, CD lookup, and other features.

Both are Windows-only.