We have a number of Galaxy S2's that we use for Serval. Some of those we keep "stock", so no root or anything fancy, for testing Serval Mesh on. The one of those I use as my day-to-day phone has recently been filling up the internal storage so that I couldn't install even small apps, even though I had very little installed on the phone.
After a bit of digging around, I discovered that this can be fixed by dialling *#9900# and then choosing the option to delete logcat. Suddenly my phone was back to having 1.6GB of free space on the internal storage :)
Hopefully this will help others who encounter the same problem.