This is one more reason why Ubuntu is the best Linux based distribution. From the 11.10 version on wards ubuntu has replaced the synaptic package manager with the Ubuntu Software Center (USC). Now the same software center is available online at https://apps.ubuntu.com . The entire catalogue of software is made available online.
Visiting this website will present you a screen with various categories of software. You can search for applications. If you are viewing this website from Ubuntu or its derivatives, You will also see an option to install the application. Clicking on the install will install the application on your system. You can also filter the applications for your Ubuntu version by choosing the version on the right side. The currently available versions are Maverick, Natty, Lucid, Oneiric. Application views look much like they do in the native desktop app, with screenshots, information and user ratings and reviews.
I've compared the performance of the online based USC and the one that is on my system. The online based software center is more faster than that is in the Ubuntu on my system.