I knew it’s there all over the web, and people are still asking, so, let’s go through it together!
Most simple Visual Studio 2005 add-ins could actually work with Visual Studio 2008. I’m not talking about huge add-ins like Resharper here, but most of the small utilities we find online. If you really want to keep using your add-in, and it doesn’t provide a VS 2008 version yet, and you didn’t have VS 2005 installed on the machine when you installed VS 2008 (as it tries to migrate the add-in when you install it in presence of VS 2005), there’s is something very simple you can try that can work for many VS2005 add-in.
Install the VS 2005 add-in. Under your "My Documents" ("Documents" in Windows Vista), you’ll see 2 folders: "Visual Studio 2005" and "Visual Studio 2008". The "Visual Studio 2005" folder will have a sub folder called "Addins". If that folder contains your add-in, and it has a file with the name of the add-in (maybe not exactly the name) and extension ".addin", then you are OK.
Just copy the add-in files (or the contents of the entire "Visual Studio 2005Addins" folder) to the "Visual Studio 2008Addins" folder. Note that "Visual Studio 2008Addins" might not exist, then you just need to create the "Addins" folder under "Visual Studio 2008" folder yourself (which all is in your "My Documents" or "Documents" folder if you forgot already!).
The last step is to open the ".addin" file with any simple text editor, like Notepad. You’ll see that it is a simple XML file that an be easily edited. Simply look for some tag called "<HostApplication>" and a child tag in it called "<Version>", and change the contents of that tag (You may find it more than once) from "8.0" (which maps to VS2005) to "9.0" (yes, your guess is right, that’s VS2008!).
Below a screenshot of the ".addin" file for the "CopySourceAsHtml" popular add-in after applying the changes:
Disclaimer: Of course I have to repeat: This should work with many VS add-ins, but it will not work with all of them.