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Update May 2014, Eclipse 4.x
As oberlies kindly points out in the comments:
Dropins are deprecated! Don't use them!
Even though they are still documented (in Kepler 4.3 for instance), this thread adds:
One of the reasons that Eclipse does not recommend the dropins folder method of installation is that there is no feedback if something goes wrong. Any number of things could be getting in the way.
- If you were using the install UI, you'd be informed of the problems before any changes are made.
- With dropins, Eclipse tries to install and start as much as it can and you don't really know what's going on.
A Local deployment, as shown in this tutorial, would rather use File → Export → Plug-in Development → Deployable plug-ins and fragments.
But the final destination remains in ECLIPSE_HOME/plugins.
Original answer (2009, Eclipse 3.x)
You should add your plugins in ECLIPSE_HOME/dropins.
If you want to defined common plugins, use bundle pools (also answered 2009-02-24T16:22:54.590