Moving between Google services

Sure! Just stay logged in and you see always your Gmail/address in the upper right hand corner...

If you registered for these services from the U.S., you should be able to move between the different services in one account (username@gmail.com).  In Gmail, you can click the link to "all my services" at the upper left.  In Google Reader, you can click "My Account" at the upper right.  Several other Google services likewise have a link to "all my services" or "My Account."  If you don't find either of these links, you can look for a link to the Google home page; once at the home page, you can click "My Account."

I do not know whether this navigation works in the same way for Google accounts registered from other countries.

Affirmative! You certainly can as long as you have a Gmail account which is positioned as a universal keys to all Google sevices. And this is the advantage of google services compare to those offered by Yahoo! Even though yahoo also has their email account as primary authorisation method you have to re-login from time to time surfing between different Yahoo services. It is so perhaps for historical reasons as Yahoo is a conglomerate of different projects acquired by Yahoo at different points and developed by different teams, hence although they are able to keep a common user database it seems they have difficulties providing end-to-end authorization

There are however some difficulties with services that were introduced by Google prior to their Gmail service. For instance you will have difficulties if you had a Google Account and registered a separate Gmail account you can not merge them, you can only add Gmail to your Google Account thus merging them together. This was a mistake I've made once myself and now I have to have both rather than just one Gmail account for everything. Here is what Google says about this.