DX cluster client with sporadic e MUF, F layer, aurora, tropo, FAI, TEP calculations and mapping / modelling and much more by G7RAU:

UFB!   http://g7rau.demon.co.uk/

Lightnings in realtime : http://www.blitzortung.org/

  http://cirrus.meteo.noa.gr/forecast/lightning.gif

  http://www.euclid.org/realtime.html

NASA Ionosphere visualization with Google Earth :

http://sol.spacenvironment.net/~ionops/ES4Dintro_public.html

Meteor radar : http://www.sgo.fi/Data/RealTime/meteorRadar.php

Meteor sky view : http://www.dl1dbc.net/Meteorscatter/

G7IZU Radio Reflection Detection http://www.tvcomm.co.uk/radio/

Radio Meteor Observatory's On Line http://www.rmob.org/pages/liveradiometeorpage.php

Meteor showers and much more in this "must have" free software : http://www.fourmilab.ch/homeplanet/

Map of rain scatter reflection points : http://tk5ep.free.fr/start.php

HAARP scientific instruments http://www.haarp.alaska.edu/haarp/data.html

Some propagation prediction tools:

W6ELprop by W6EL http://www.qsl.net/w6elprop/

HAMCAP by VE3NEA  http://www.dxatlas.com/hamcap/

Propagation by K6TU http://www.k6tu.net/

GREAT Propagation bulletins by K7RA  http://www.arrl.org/w1aw-bulletins-archive-propagation

and the best VOACAP by OH6BG/OG6G  http://www.voacap.com/