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Im able to set how many colours in gifs, the quality of jpegs, and a lot more. 'Save for as web' is nice too... it lets me see a few different settings at once, and see the file size vs quality etc.
photoshop by far has to be my most valuable tool. =)
It has the ability to really crunch images before you put 'em out there.
You should be able to get trial versions of all the best optimising software free with magazines or over the web - have a search through the DMOZ [dmoz.org...]
DeBabelizer used to be a pretty well-known program on Mac, but a lot of the best optimizers/compressors I've seen recently are Photoshop plug-ins (which a lot of other programs can use).