If you are regularly downloading articles or documents to read that are in PDF, you can use the built in screen reader in Adobe Acrobat Reader called Read Out Loud to hear any text in the document read aloud.
If you need assistance with reading MS Word documents, or to have the text of your writing read back to you for editing purposes, one good option might be WordTalk, a free program text-to-speech program for Microsoft Word. You can also use the built-in screen reader/text-to-speech features on your computer.
Both Microsoft and Apple have simple text-to-speech programs built into their operating systems. Microsoft's Narrator is relatively limited in features, and is intended for users with visual impairments. However, some of the features may be helpful for you. Apple's VoiceOver has similar capabilities and can assist users with reading typed text, windows, menus and controls.
If you are mostly concerned with being able to hear text on websites read aloud, you might consider ROKTalk. ROKTalk is a web-based application that allows you to hear any web text read out loud. Because it is web-based, you don't have to install software, which may be useful if you are using different computers (at the library, in the classroom, etc.). ROKTalk is free for a few websites (Google, Wikipedia, the BBC) and charges a monthly fee for unlimited access to any website. Browser-based apps such as Google Chrome's text-to-speech tool, SpeakIt! and Read&Write for Google Docs are also effective tools for online reading. Similarly, Natural Reader has free text-to-speech programs with limited functionality that may be sufficient for your needs. Several options are available from a basic copy for free download, to a more full-featured version for purchase.
If these free tools don't provide you with the level of functionality you need, you can also try searching the TechMatrix to find other text-to-speech products and compare features by selecting the Subject Area of Reading and the Learning Support of Access to multiple formats of text, notation, and symbols.