Garmin Forerunner 610 and a heart rate band. (Now this heart rate calculator might come in handy.)
One of my favorite features about the 610 is that it will scroll to different pages with new information so that I don't have to tap the screen to see new information. I can use it with sweaty fingers. It's smaller than my Garmin 310XT. The battery life is about 8 hours so if I ever do an ultra (you never know!), I'll use my Garmin 310XT.
I ran 18 miles in the heat yesterday and had no cramps, chafing or blisters (used Body-glide and Thorlo Experia socks) and no soreness today (I don't seem to have soreness after most of my long runs now). I ate 4 GU gels and took two salt tabs because it was 75-85° and humid. Pouring cold water on my head and neck cooled me off and I kept the pace easy.