ING Direct has long made it easy for a customer to add additional savings accounts. It's also possible, but a little more difficult, at Ally Bank. This NYT Bucks blog post
has a good review of why you may want to open multiple savings accounts at your internet bank, and it has links to examples of how to do this at ING Direct and at Ally Bank. It also mentioned that FNBO Direct has made it easier for customers to do this.
In addition to helping you save for different things, there is also an advantage if you need to make more than 6 withdrawals per month. Each savings account should allow up to 6 withdrawals per month. Be sure to check with your bank to confirm this. In the past I received confirmation from both ING Direct and Ally. So if you have two savings accounts, you should be able to do a total of 12 withdrawals per month. I don't believe banks are allowed to create separate accounts for this purpose, but it's a side benefit.