Lego Digital Designer

Lego digital designer, available at ldd.lego.com, allows you so design and order your own Lego models on the computer. This should be available as a portable app.

From their site:

Disclaimer
The LEGO Group of companies (LEGO) is providing LEGO® Digital Designer 1.6 (the Software) at no charge. Any commercial use of the software is strictly prohibited. The entire risk arising out of the use or performance of the software remains with you. The software is provided as-is without any warranty of any kind. To the maximum extent permitted by applicable law, in no event shall LEGO be liable for any damages arising out of the use or inability to use the software. To install and use the software, you must agree to the terms of the LEGO Digital Designer 1.6 License Agreement included with the software. Please be sure to read the License Agreement before installing LEGO Digital Designer 1.6 on your system.

Also from their site:

Minimum System Requirements for PC
Operating system:
Windows 98, 98SE, ME, 2000 and XP.
CPU: 450 MHz Pentium III or faster processor
Graphics card:
16 MB graphics card
(DirectX 8.0 or higher compatible)
RAM: 128 MB
Hard Disk Space: 100 MB

Minimum System Requirements for Mac
Operating system:
OS X 10.3.9 or 10.4.6
CPU: G4 or faster processor
Graphics card:
NVidia GeForce 5200/ATI Radeon 7500 or better
RAM: 256 MB
Hard Disk Space: 100 MB

can't find the licence agreement, though. considering that their last software (mindstorms nxt) had a "right to hack" in the licence...