I have used a simple script called phpThumb (not a WP plugin, but an external script) which works great.. It is WP friendly and can be called from any image tag. Its is fine for single images, however is not a good solution for entire galleries..
I have tried one called “duh Gallery” which I thought was great until I realized it was force-resizing rather than using GD.
I tried combining these two scripts to make an image thumnailing gallery but it was horribly innefficient and killed my server.
Hopefully in later versions this functionality will be available.
I am interested to see if anyone here has found any better scripts for this