Gamescience and Easy Roller Dice Company is a close second.
The thing is that lately I've been playing in online games where the GM prefers we all use the Roll20 dice roller. This is great when everybody is online, but when you need online dice when "offline" from the group?
Enter the Play By Email Dice Roller. Yesterday I went to finish rolling up a backup PC and wanted a way to roll online without spamming the group chat. Using this dice roller you can have up to five "rolls" (can be multiple dice in each roll) which can be shared with up to five email addresses and......and there is a built-in verification system. The results can be emailed, displayed on the screen, or both. I only saw two minor issues with the dice roller, and one was due to the system I was playing. You see the results, not the roll itself, so 3d4+1 would get you a result ranging from 4 to 13. If you needed to see the results of each d4 you'd just have to tweak the roll a bit.
The other minor issue was just me being stupid and selecting "display" instead of "both".