AdBlock does a good job on these. I have meebo.com and wibiya.com completely blocked, as well as facebook.com, fbcdn.net, tynt.com, etc. I use a different browser installation altogether for things like Facebook when I actually want to download their stuff.
Basically, any time a website takes ages to come up, I turn on Firebug's Net panel and monitor the laggy requests; and then, if warranted, kill them off with AdBlock. Similarly for offensive widgets.