First, a warning. By doing this you are allowing software to be installed on yor phone that could be from anywhere. Similar to how you might download programmes from the web to your PC, it is okay, but you just need to be sensible about where you get them from.
Anyway, this isn't completely detailed (I have assumed small things like you know how to use an SD card) but here goes...
In summary >> Find an app file on the web >> Copy it to your SDcard >> Use AppManager to install it on your phone >> job done
1. First you need an App Installation programme. You can find these in the market - try AppManager or Astro. I prefer AppManager as it is fairly easy. This app lets you copy apps from your phone memory to your SD card. It will also install apps from your SDcard to your
2. You need to find an .apk file for the application you want. This file installs an app on your phone. You can try searching using Google e.g. Barcode scanner android .apk, >> or going to androlib.com which has links to official websites for the developers, many of which have the .apk file on their websites, or you can ask here in these forums where to find something. (BUT, free apps only please). Save it to your PC desktop or somewhere.
3. Connect your phone to your PC using the data cable. Copy the .apk file to your SD card into the folder used by app installer application (AppManager uses a folder called appmanager). Then disconnect your phone.
4. Run AppManager (or other app installer) and install the app. To install the app In AppManager you select Menu > Install then select the app you want. It will give you a warning and ask you to change a setting (Settings>Application Settings>Allow Unknown Sources).
Alternatively, you can download direct from soime websites to your phone (I think). Or use an alternative to the market such as App Store available from andappstore.com
I've done this a lot as so many apps don't have the right screen settings in Market, but do actually work.
Hope this helps.
- Mark