I use the excel­lent Sub­merge to flat­ten sub­ti­tles into my AVI files and other movies. But some­times, sub­ti­tles are avail­able from sources such as Sub­Scene or All­Subs as not the usual *.srt files (Sub­Rip for­mat) but as *.ssa or *.sub.

While search­ing for a con­verter between the cap­tioned for­mats, Google did not turn up too much. After some search­ing I found a nifty lit­tle util­ity that allows you to con­vert between many for­mats of sub­ti­tles. It's Win­dows only, so if you are not on Win­dows you may have to use this within a vir­tual machine:

Down­load Sub­ti­tle Workshop

Usage is as sim­ple as load­ing a sub­ti­tle file from the File menu, and then Save As and Bob's your uncle.