If you want to leave a message for yourself, to record a quick thought or a short brilliant idea, you can use Google Talk and Gmail and have it available online.
How can we do that? Gmail supports the standard "plus" addressing scheme. The "plus" method of addressing lets you add additional words to your account name (the "left side" of your email address.)
So, add new "friend" with email address email@example.com to Google Talk. (Don't worry if he's never accepted your invitation. :-)) Use it every time when you want so send voicemail to yourself. To keep things tidy, you can create label AnsweringMachine in Gmail and filter all your voicemails. You can even set filter to forward it to another email address as an email notification so you don't forget. There is also a "hidden" built-in label "voicemail" (search for label:voicemail) that you can use if you don't want some special filtering.