Some others in addition to what Tyler mentioned:
College of Dupage: on any model map image, just click on the map and it will launch a sounding for that time http://weather.cod.edu/forecast/
TwisterData: similar to Dupage, just click on the map - http://www.twisterdata.com/
Others for sounding or sounding data:
http://www.wxcaster.com/models_text.htm
http://vortex.plymouth.edu/myo/fx/raobfx.html
http://www.stormchaser.niu.edu/machine/textfcstsound.html
For observed soundings (not modeled): http://www.spc.noaa.gov/exper/soundings/