i love this question, almost because i know exactly the answers that has been answered tons of time in this forum..
TS - be prepared, you will get lots of pointers to "search" function
1) If u have budget - get SB900
2) If not and want to experiment and learn with flash - get SB600
3) Shoe string budget and flash only to be used for internal shooting and fill flash, where "just a flash" will do - get SB400
SB400 is very small, smaller than the cell u hold and is very nice to carry when travelling but has few restrictions on usage. SB900 has all the fancy stuff with wireless and Creative Lighting system that is good if you want to use them but is expensive and heavy. SB600 is just right if you want to learn and experiment.
For me, i bought SB400 not because of budget but i was sure it would serve my purpose and the size is the one thats very good for me plus its light. I use it indoors for bounce. Already can see the effect in pics, much better than the built-in flash.
My advice to all, ext flash is an absolute MUST, if you want something very simple go for SB400, its very cheap and you will be amazed the way it improves your indoor pics.
Please define "good" and let us know. You can also check the respective websites for further details about the flash models mentioned by others and adding "review" to the model plus entering the string in Google gives plenty of information.