As of about a month ago there was already 80,000 iPhone Apps!
Josh Clark has done us all a favour, at least for the moment, as this has to date fairly quickly, by writing a guide to the best Apps. I got a copy of his book in Kinukuniya (Emporium) yesterday and have downloaded about 20 of the top 200 he recommended. Good stuff. Many are free. (Some of the “location based” ones don’t work in Thailand yet, but some do).
If you want to save yourself 500-600 baht, then Josh has done a film (not great audio quality) on youtube about some of the better Apps. This is part 1, part 2 is also on youtube (in fact the second film below). If your time is more precious than the money, you might just want to get the book and share it among friends.