Perched high atop this list is my .Mac account. I get so much out of this $99 per year that it's just unreal! Web-based email saves my rear when I lose the "I-can-receive-email-but-I-can't-freaking-send-email" battle with a hotel. File transfer via iDisk works great for sharing those big, honking files that are sure to choke most email accounts. It's also good for those who are uncomfortable with using an FTP (file transfer protocol) client (such as Fetch), as accessing anyone's Public folder is really simple through the Finder. I also use my .Mac account for publishing web pages of photos on my trips, and of course, for sending those cute and fun iCards.
Most Starbucks (mmmm... cafe mocha), airports, and book stores have wireless Internet access available through T-Mobile hot spots. This is a great account to have; however, beware of the payment plan that you are on! Be very careful as you don't want to rack up a bunch of "per minute" charges. Note: If you sign-up for a T-Mobile account with a promotional 24-hr free thing, you *cannot*, I repeat *cannot* change that account to any other plan. Once the 24 hours are up, you are automatically on a Pay As You Go plan which means they bill you by the minute. Very scary indeed.
Need to receive a fax while on the road? It's no problemo with eFax service. Once you sign up for this service (it's free for up to 20 pages every 30 days) you can get a faxed delivered straight to your email in box in digital form. Can't beat that! It's still cheap if you exceed 20 pages: $12.95 per month, and $139.95 per year.