Slang.gr is a truly great site about Greek slang. It covers not only colloquial expressions, but also jargon used in various other spheres such as: military service, sports, love etc. etc.
Now, what would life be like if we didn't have to deal with grammar? This is a nice blog for conjugation ot modern Greek verbs. It's quite well organized and together with this Comprehensive list of modern Greek verbs' conjugation and translation into English, it may be the absolute reference for learners of Greek as a foreign language.
And last but not least, don't forget to check out once in a while my friends at GreekArgo.blogspot.com who have a new interesting expression and a funny illustration to offer every week at their wonderful site.