Eclipse might not have any up-front monetary costs, but it has costs in lost productivity while waiting on it (which would arguably cost you more than a closed source license when applied to the value of a developer's time).
It's gotten better, but it still has a ways to go.
My machine is beefed up so I'm sure someone else's cost using less powerful machine (Intel P3 or something...) is more than mine. Ditto with VS.NET 2010 or future version of Microsoft IDE when used in older machines.
Shall we count the cost of using plugins as well? Including OS boot time and such and such?
Sorry, what I mean to say is that I don't know what you're talking about of this "waiting on it".
There's also IntelliJ, an IDE from the guys that make Resharper. We moved from Eclipse to IntelliJ a few years ago and aren't looking back. It "just works", has (had?) more reliable and advanced refactoring, and the support for other languages (like JavaScript, Python and Ruby refactoring and debugging) is/was far ahead of Eclipse.
Most people use VS.NET with Resharper while Eclipse has a few Resharper features built-in and more.
Eclipse + Maven = great deal.