Gmail shows more links in the navigation bar from the top left corner. If you use Picasa Web Albums and Google Spreadsheets, you'll see links to these services. There's also a link to "all my services". Google has yet to find a way to make it easy to move from one service to another. One idea is to let people select a number of favorite services that will appear on the homepage and on other pages.
We'll see more integration with Gmail in the future: you'll be able to send photos from your albums using Gmail or attach Excel documents from Google Spreadsheets.
Update: Google Spreadsheets is now a public beta, so you can just use your Google Account to log in.