Or if you want an advanced screenshot taker which let's you do hell lot of things other than bringing up a simple GUI, then you should try the Shutter. Few hours ago they announced the 0.87.2 version, which brings a feature called "Quick lists", it is primarily focused on giving a better user experienced in the upcoming Unity desktop environment (see below screenshot)
|"quicklist", this is what it means ;-)... (Only for Unity)|
To extent its features, you can install plugins which adds adding extra effects to pictures, etc as well. But most importantly, Shutter makes it pretty easy to share those screenshots with ease. If you want to quick edit the captured images, then Shutter equips with a simple image editor too!.
|Shutter in action!|
Although when you first install this new version, due to some complications with an applet called "libappindicator" which is the reason for the recent Gnome and Canonical conflicts thus giving birth to the Unity desktop, anyhow, currently on Ubuntu 11.04, the Shutter icon which should be shown in the notification area won't simply be there!. But luckily there is a way to fix that once you install (only happens in 11.04) it.
So first install Shutter on Ubuntu PPA, as usual, open your terminal and issue the following command.
sudo apt-add-repository ppa:shutter/ppa
sudo apt-get update
sudo apt-get install shutter
If you want to get that "missing" notification icon for Shutter (need to be done for Natty only), then read this OMGUbuntu article which explains it in a very simple manner.