Google Talk's voicemail feature allows your friends and contacts to send a voice message to your Gmail inbox. Voicemails show up as special messages in your inbox, where Gmail's built in Flash voicemail player lets you listen to them without having to launch an external application - just click the Play button in the conversation view.
Voicemails come into Gmail with the subject Voicemail from contact (x seconds) and are tagged with a telephone icon in the attachments column.
You can also search for voicemails in Gmail using is:voicemail.
Learn more about Google Talk.