Dallas
I highly recommend the Nelson-Tebedo Health Resource Center for STD testing. They offer an STD testing package that screens for HIV, syphilis, gonorrhea, chlamydia, and hepatitis A, B, and C for $100. $100! That is super cheap. Herpes simplex 1 & 2 testing is extra, but it's also low cost. They also offer hepatitis B vaccinations for $10 each (3 injections total over 4 - 6 months are needed).
The staff is very friendly (some work there as volunteers) and are extremely respective of anonymity. Cash payments are welcome. 
A post on GD reminded me I should spread the word about this place. I wish I'd known about it sooner -- would have saved me a lot of money! I recommend calling ahead and scheduling an appointment unless you want a really long wait time.