Because emails are stored in your inbox, it sort of becomes "visual" voicemail by default. In other words, if you scan the contents of your inbox, you can see the voicemail you just received at the top of your inbox, but still choose to play the one received several hours ago, first.
According to HulloMail, "A key feature is the fact that it works free of charge with your existing mobile number and your existing service, which Google [Voice] currently does not... HulloMail voice-email service is fully automated and free, and offers full syncing capabilities with IMAP4 email accounts, so you can access it from anywhere and on virtually any device."
HulloMail already works with Android-based devices, and HulloMail says that an iPhone version is on the way soon. HulloMail also says that it works with traditional landlines.
HulloMail is pitching itself as a great app for business users to take advantage of, too. It is compatible with CRM systems, creates instant email logs, uses SSL for security, is compatible with other voicemail systems, and lets end-users easily manage their messages.
It seems like a nice service to have, and lets you avoid a monthly fee (charged by some wireless operators) for visual voicemail services. It's hard to argue with free. Users of the Blackberry Bold, Blackberry Tour, and Blackberry Curve 8900 models can download the application here.
Here is a video demonstration of how it works: