I took my NREMT-B last month and used Prentice Hall's EMT Achieve tests for practice and passed. I recommend EMT Achieve because it simulates the questions on the actual test and it provides explanations for each individual question, whether you got it right or wrong. Even though I felt like the questions on the actual test were a tad more difficult than EMT Achieve's, I still feel that it best prepped me for the NREMT-B exam.
EMT Achieve: http://myphlip.pearsoncmg.com/alttestprep/emtbasicbridgepage.html
Note: I only have experience with the EMT-B practice tests, not the paramedic or first responder ones.