If you're running Snow Leopard, one of the more important things that breaks is support for Input Manager-based plug-ins, such as (amongst many others) SafariStand, PithHelmet, Chax, MailTags, Mail Act-on, etc. In the long term, authors of these plug-ins are going to have to find another way to make their plug-ins work. For instance, both 1Password and Chax already have Snow Leopard-compatible solutions available.

In the short term, however, if you're willing to change the way your plug-in-using applications run in Snow Leopard, you can continue to use the Input Manager-based add-ons. To do this, quit the program in question, then click on it in the Applications folder and press Command-I to open the Get Info window. In the General section of the Get Info window, you should see a checkbox labeled Open in 32-bit mode. Check that, then close the Get Info box and reopen the application.

This should let any Input Manager-based add-ons work with the program in question, though I haven't done enough testing to claim it works for everything. I've chosen to run this generic hint about Input Managers in Snow Leopard, as opposed to a series of very similar hints on enabling each given Input Manager-based add-on. If you can further clarify the usability of this solution for specific add-ons, please feel free to do so in the comments.