Spring and especially summer are the time of the tourists' "flow" into Moscow, as into any other country...
In the Internet you may find plenty of suggestions of tourfirms: professional guides will provide you with essential information concerning history, culture, traditions of the city.
In this very post we would like to mention an organisation called "Moscow Greeter".
Now this organisation is free: volunteers, among them mostly students of linguistic universities, will show you the city from their viewpoint and will tell you everything they know. That's why, don't expect professional guides, however, friendly atmosphere is guaranteed.
The official website is http://www.moscowgreeter.ru/index.html
On should sinmply fill in the form.
Have a good travel in Moscow!