Zamzar is one of the best, freely available service that helps you convert your text documents to mp3 format.
Some sharewares available in market are:
1> Yeosoft Text to MP3 Speaker:
Beauty of this software is that you can translate the content into 10 different languages (English, French, German, Russian, Dutch, Spanish, Japanese, Portugese, Italian and Korean).
You can download a trail version for free, and buy the license for $19.95
2> Alivemedia’s Text Speech is a good option.
It makes use of the microsofts text to speech engine, and the text can be converted to various formats like MP3, WAV, Vox, OGG files.
It is compatibale with Windows xp and also with Windows Vista.