I have always been a fan of the Seti@home (Search for Extraterrestral Intelegence) effort. and recently decided to get back into it. what I found is that they now have a easy way for you to donate your computer time to all sorts of scientific endevors.
it uses you space computer cycles to run all sorts of distributed computing projects you can attach to. and most of those have nice screen savers.
one word of advice though. pause it before you play flight simulator. and if you have a pentium 4 like me turn down the amout of processor time it uses unless you want your computer to sound like a vacume cleaner with the fan running all the time.
seti@home actually was the start of all of this. they went trough several different software programs and then decided to try to make one that would make it easyer for users to work for multiple projects. a very tangeble result for a project that is other wise somewhat out there.
of course it goes without saying that I will be runing my antivirus software regurlary. but this has about as many credenturals as you can have for something like this. the client instal even has a certificte.
If there is anything out there, do you really think it's sensible to advertise our presence? If they are more intelligent than us they they will already know about us and have decided to avoid us. If they are less intelligent than us then they will not understand our signals and will not be able to reach us anyway. (If they are of equal intelligence then the same applies ).
And isn't it naive to assume that they will be friendly? Would you walk through Central Park at night flashing a torch and saying hallo anybody want to be my friend?