RPThreadTracker
https://www.rpthreadtracker.com
I cannot stress enough how helpful this site is. it is completely free and if you are as anal as me about organisation and if you are worried about tumblr eating your notifications when you get replies, then this site could help you.
it lets you know which threads are the ones where it is your turn to reply and which ones are the ones where your partner has yet to reply to. there is even a random thread picker to help you pick one to reply to.
you can add as many character as you want, which could be useful for multi-muse blogs and to help you keep track of what thread had which muse involved. you can add tags to tracked threads so you know which au or version this thread was using too!
To add a thread just click the plus sign at the top.











