As the name implies, you can use Gmail Offline even with intermittent Web connectivity: It can connect to the service, synchronize messages, and disconnect so you can keep working on the plane or wherever you don't have Net access. But even with a stable, constant connection, Gmail Offline can serve you as a powerful alternative to a desktop email client. For one thing, it has Gmail's powerful filters, which can help you tame your inbox by automatically organizing incoming email; you configure the filters using the regular Gmail interface. You can also send mail from multiple From addresses, and it supports many of Gmail's regular keyboard shortcuts.
If you prefer to keep your email with Microsoft instead of Google, you could use the Outlook.com Notifier extension to quickly see how many email messages are waiting in your Outlook.com (formerly Hotmail) inbox. This simple extension appears as an icon with a counter overlay showing the number of unread messages, and it's smart enough to use Chrome's Desktop Notifications feature to display a pop-up whenever you receive a new message (you can disable this function if you don't appreciate the constant distractions).
Spreadsheets: Google Spreadsheets and Zoho Sheets
Replacing Excel, the de facto standard for spreadsheets, is not as easy as replacing your word processor or email client. Yes, Google Drive also offers spreadsheets, but there the difference in power and features is much more pronounced. For a quick example, just open a new Excel spreadsheet on your computer and hold the Page Down button. You could be scrolling down for minutes: In Excel 2013, I was able to get to over 100,000 rows. With Google's Spreadsheets, you'll run out after less than a second, since it tops out at 100 rows by default. If you want more rows, you have to add them manually by clicking a button at the bottom of the spreadsheet -- and you get just 20 rows at a time, though you can change that pretty easily. Another Web-based spreadsheet, Zoho Sheets, handles the limited-row issue more elegantly, transparently adding rows as you scroll down.