Radio and television stations, advertising agencies, talent scouts, companies looking for someone to voice their automated attendants and voice mail systems for their business telephone systems, book authors that want an audio book version, etc. Basically, anyone who would want a professional voice to represent their product or service on radio, television, internet, CD, DVD or other broadcast or static media.
In case you don't know what voiceover is, Wikipedia sums it up pretty well...
The term voice-over refers to a production technique where a professional voice is broadcast live or pre-recorded in radio, television, film, theatre and/or presentation. The voice-over may be spoken by someone who also appears on-screen in other segments or it may be performed by a specialist voice actor. Voice-over is also commonly referred to as "off camera" commentary.
The term voice-over can also refer to the actual voice actor who performed the recording. The terms voice actor, narrator, voice artist, and announcer are all similarly used.