Another small but useful word here, ‘いがい／意外／igai’、loosely meaning ‘unexpected’ or ‘surprising’, which describes situations I seem to find myself in more than I probably should.
Slightly more mature than ‘しんじならない’/shinji naranai) which is closer to ‘I don’t believe it’, it becomes useful in situations when dealing with certain business people ;). A basic use is given here:
“問題は意外にやさしかったよ.” – I found the problem easier than I had expected.
For more examples, as ever, check out the relevant WWWJDIC page here. Goo dictionary definition here.