If you are looking to only fill in existing forms within PDF files, then the free Adobe Acrobat Reader will work perfectly. Another alternative is the free FoxIt PDF Reader. It's faster than Adobe Reader and bloatware free.
However, if you're looking to create PDF forms, then FoxIt PDF Creator would be your best bet. It's full featured, supports most functions included in Acrobat Pro (see note below) and only costs $29.99 USD.
http://www.foxitsoftware.com/pdf/creator/
Edit: I forgot to mention that if you are looking for software that supports virtually every feature included in Acrobat Pro and allows you to edit existing PDFs, FoxIt PDF Editor would be your best bet...but it costs $99 USD