I have the Topeak Large Wedge Seat Pak for when I really want to carry a bunch of stuff (not often). It's expandable too and includes a waterproof cover that stows neatly in a seperate little pouch. There's also bungee straps that can be used to secure things like a jacket. You can get an idea of the size of it from this picture of it mounted on my MTB.

For most normal rides, I use the smaller Small Wedge Seat Pak. It isn't expandable and is just big enough to hold my minitool and small bottle of sunscreen or my small mobile phone. It does have bungee straps on the bottom which I use to secure a spare tube. Here's a picture of it attached to the saddle of my roadbike

They have a medium sized one too. Also you can get them in "aero" versions. I like them because they're easy to mount and dismount, don't shift around, can carry just the right amount of stuff and they're plenty sturdy.